Add an auto-instrumentable no-op implementation to the trace package (#6203)

This copes the `go.opentelemetry.io/auto/sdk` package into the
`go.opentelemetry.io/otel/trace` package. This is done to avoid package
import cycles and still provide an auto-instrumentable SDK (see
https://github.com/open-telemetry/opentelemetry-go-instrumentation/issues/974).

## Overview of changes

The code copied is updated with the following changes. The over-all goal
is to ensure none of this is exported and follows the `auto/sdk` as
close as possible to help maintenance.

### `trace/auto.go`

Consolidation of the following into a single file:

-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/tracer_provider.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/tracer.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/span.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/limit.go

Has the following changes:

- `func TracerProvider()` renamed to `newAutoTracerProvider`
- `type tracerProvider struct` renamed to `autoTracerProvider`
- `type tracer struct` renamed to `autoTracer`
- `type span struct` renamed to `autoSpan`
- Lint issues addressed based on this repositories configuration (these
changes are being back-ported upstream)

### `trace/auto_test.go`

Consolidation of the following into a single file:

-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/tracer_provider_test.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/tracer_test.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/span_test.go
-
https://github.com/open-telemetry/opentelemetry-go-instrumentation/blob/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/limit_test.go

Has the following changes:

- Renames in `trace/auto.go` are applied here
- Lint issues addressed based on this repositories configuration (these
changes are being back-ported upstream)

### `trace/internal/telemetry`

Copied from
https://github.com/open-telemetry/opentelemetry-go-instrumentation/tree/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/internal/telemetry

- Pacakge vanity URLs added
- Lint issues addressed based on this repositories configuration (these
changes are being back-ported upstream)
- Use of the package name has been updated

#### `trace/internal/telemetry/test`

Copied from
https://github.com/open-telemetry/opentelemetry-go-instrumentation/tree/aea085dd2a3640630ac07a2187cbda9d15d2dd00/sdk/internal/telemetry/test

- Module name updated
- Documentation updated with new package name
- Testing values updated with new package name

---------

Co-authored-by: Ron Federman <73110295+RonFed@users.noreply.github.com>

trace/auto.go

@@ -0,0 +1,661 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package trace // import "go.opentelemetry.io/otel/trace"
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"math"
+	"os"
+	"reflect"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+	"unicode/utf8"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
+	"go.opentelemetry.io/otel/trace/embedded"
+	"go.opentelemetry.io/otel/trace/internal/telemetry"
+)
+
+// newAutoTracerProvider returns an auto-instrumentable [trace.TracerProvider].
+// If an [go.opentelemetry.io/auto.Instrumentation] is configured to instrument
+// the process using the returned TracerProvider, all of the telemetry it
+// produces will be processed and handled by that Instrumentation. By default,
+// if no Instrumentation instruments the TracerProvider it will not generate
+// any trace telemetry.
+func newAutoTracerProvider() TracerProvider { return tracerProviderInstance }
+
+var tracerProviderInstance = new(autoTracerProvider)
+
+type autoTracerProvider struct{ embedded.TracerProvider }
+
+var _ TracerProvider = autoTracerProvider{}
+
+func (p autoTracerProvider) Tracer(name string, opts ...TracerOption) Tracer {
+	cfg := NewTracerConfig(opts...)
+	return autoTracer{
+		name:      name,
+		version:   cfg.InstrumentationVersion(),
+		schemaURL: cfg.SchemaURL(),
+	}
+}
+
+type autoTracer struct {
+	embedded.Tracer
+
+	name, schemaURL, version string
+}
+
+var _ Tracer = autoTracer{}
+
+func (t autoTracer) Start(ctx context.Context, name string, opts ...SpanStartOption) (context.Context, Span) {
+	var psc SpanContext
+	sampled := true
+	span := new(autoSpan)
+
+	// Ask eBPF for sampling decision and span context info.
+	t.start(ctx, span, &psc, &sampled, &span.spanContext)
+
+	span.sampled.Store(sampled)
+
+	ctx = ContextWithSpan(ctx, span)
+
+	if sampled {
+		// Only build traces if sampled.
+		cfg := NewSpanStartConfig(opts...)
+		span.traces, span.span = t.traces(name, cfg, span.spanContext, psc)
+	}
+
+	return ctx, span
+}
+
+// Expected to be implemented in eBPF.
+//
+//go:noinline
+func (t *autoTracer) start(
+	ctx context.Context,
+	spanPtr *autoSpan,
+	psc *SpanContext,
+	sampled *bool,
+	sc *SpanContext,
+) {
+	start(ctx, spanPtr, psc, sampled, sc)
+}
+
+// start is used for testing.
+var start = func(context.Context, *autoSpan, *SpanContext, *bool, *SpanContext) {}
+
+func (t autoTracer) traces(name string, cfg SpanConfig, sc, psc SpanContext) (*telemetry.Traces, *telemetry.Span) {
+	span := &telemetry.Span{
+		TraceID:      telemetry.TraceID(sc.TraceID()),
+		SpanID:       telemetry.SpanID(sc.SpanID()),
+		Flags:        uint32(sc.TraceFlags()),
+		TraceState:   sc.TraceState().String(),
+		ParentSpanID: telemetry.SpanID(psc.SpanID()),
+		Name:         name,
+		Kind:         spanKind(cfg.SpanKind()),
+	}
+
+	span.Attrs, span.DroppedAttrs = convCappedAttrs(maxSpan.Attrs, cfg.Attributes())
+
+	links := cfg.Links()
+	if limit := maxSpan.Links; limit == 0 {
+		n := int64(len(links))
+		if n > 0 {
+			span.DroppedLinks = uint32(min(n, math.MaxUint32)) // nolint: gosec  // Bounds checked.
+		}
+	} else {
+		if limit > 0 {
+			n := int64(max(len(links)-limit, 0))
+			span.DroppedLinks = uint32(min(n, math.MaxUint32)) // nolint: gosec  // Bounds checked.
+			links = links[n:]
+		}
+		span.Links = convLinks(links)
+	}
+
+	if t := cfg.Timestamp(); !t.IsZero() {
+		span.StartTime = cfg.Timestamp()
+	} else {
+		span.StartTime = time.Now()
+	}
+
+	return &telemetry.Traces{
+		ResourceSpans: []*telemetry.ResourceSpans{
+			{
+				ScopeSpans: []*telemetry.ScopeSpans{
+					{
+						Scope: &telemetry.Scope{
+							Name:    t.name,
+							Version: t.version,
+						},
+						Spans:     []*telemetry.Span{span},
+						SchemaURL: t.schemaURL,
+					},
+				},
+			},
+		},
+	}, span
+}
+
+func spanKind(kind SpanKind) telemetry.SpanKind {
+	switch kind {
+	case SpanKindInternal:
+		return telemetry.SpanKindInternal
+	case SpanKindServer:
+		return telemetry.SpanKindServer
+	case SpanKindClient:
+		return telemetry.SpanKindClient
+	case SpanKindProducer:
+		return telemetry.SpanKindProducer
+	case SpanKindConsumer:
+		return telemetry.SpanKindConsumer
+	}
+	return telemetry.SpanKind(0) // undefined.
+}
+
+type autoSpan struct {
+	embedded.Span
+
+	spanContext SpanContext
+	sampled     atomic.Bool
+
+	mu     sync.Mutex
+	traces *telemetry.Traces
+	span   *telemetry.Span
+}
+
+func (s *autoSpan) SpanContext() SpanContext {
+	if s == nil {
+		return SpanContext{}
+	}
+	// s.spanContext is immutable, do not acquire lock s.mu.
+	return s.spanContext
+}
+
+func (s *autoSpan) IsRecording() bool {
+	if s == nil {
+		return false
+	}
+
+	return s.sampled.Load()
+}
+
+func (s *autoSpan) SetStatus(c codes.Code, msg string) {
+	if s == nil || !s.sampled.Load() {
+		return
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.span.Status == nil {
+		s.span.Status = new(telemetry.Status)
+	}
+
+	s.span.Status.Message = msg
+
+	switch c {
+	case codes.Unset:
+		s.span.Status.Code = telemetry.StatusCodeUnset
+	case codes.Error:
+		s.span.Status.Code = telemetry.StatusCodeError
+	case codes.Ok:
+		s.span.Status.Code = telemetry.StatusCodeOK
+	}
+}
+
+func (s *autoSpan) SetAttributes(attrs ...attribute.KeyValue) {
+	if s == nil || !s.sampled.Load() {
+		return
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	limit := maxSpan.Attrs
+	if limit == 0 {
+		// No attributes allowed.
+		n := int64(len(attrs))
+		if n > 0 {
+			s.span.DroppedAttrs += uint32(min(n, math.MaxUint32)) // nolint: gosec  // Bounds checked.
+		}
+		return
+	}
+
+	m := make(map[string]int)
+	for i, a := range s.span.Attrs {
+		m[a.Key] = i
+	}
+
+	for _, a := range attrs {
+		val := convAttrValue(a.Value)
+		if val.Empty() {
+			s.span.DroppedAttrs++
+			continue
+		}
+
+		if idx, ok := m[string(a.Key)]; ok {
+			s.span.Attrs[idx] = telemetry.Attr{
+				Key:   string(a.Key),
+				Value: val,
+			}
+		} else if limit < 0 || len(s.span.Attrs) < limit {
+			s.span.Attrs = append(s.span.Attrs, telemetry.Attr{
+				Key:   string(a.Key),
+				Value: val,
+			})
+			m[string(a.Key)] = len(s.span.Attrs) - 1
+		} else {
+			s.span.DroppedAttrs++
+		}
+	}
+}
+
+// convCappedAttrs converts up to limit attrs into a []telemetry.Attr. The
+// number of dropped attributes is also returned.
+func convCappedAttrs(limit int, attrs []attribute.KeyValue) ([]telemetry.Attr, uint32) {
+	n := len(attrs)
+	if limit == 0 {
+		var out uint32
+		if n > 0 {
+			out = uint32(min(int64(n), math.MaxUint32)) // nolint: gosec  // Bounds checked.
+		}
+		return nil, out
+	}
+
+	if limit < 0 {
+		// Unlimited.
+		return convAttrs(attrs), 0
+	}
+
+	if n < 0 {
+		n = 0
+	}
+
+	limit = min(n, limit)
+	return convAttrs(attrs[:limit]), uint32(n - limit) // nolint: gosec  // Bounds checked.
+}
+
+func convAttrs(attrs []attribute.KeyValue) []telemetry.Attr {
+	if len(attrs) == 0 {
+		// Avoid allocations if not necessary.
+		return nil
+	}
+
+	out := make([]telemetry.Attr, 0, len(attrs))
+	for _, attr := range attrs {
+		key := string(attr.Key)
+		val := convAttrValue(attr.Value)
+		if val.Empty() {
+			continue
+		}
+		out = append(out, telemetry.Attr{Key: key, Value: val})
+	}
+	return out
+}
+
+func convAttrValue(value attribute.Value) telemetry.Value {
+	switch value.Type() {
+	case attribute.BOOL:
+		return telemetry.BoolValue(value.AsBool())
+	case attribute.INT64:
+		return telemetry.Int64Value(value.AsInt64())
+	case attribute.FLOAT64:
+		return telemetry.Float64Value(value.AsFloat64())
+	case attribute.STRING:
+		v := truncate(maxSpan.AttrValueLen, value.AsString())
+		return telemetry.StringValue(v)
+	case attribute.BOOLSLICE:
+		slice := value.AsBoolSlice()
+		out := make([]telemetry.Value, 0, len(slice))
+		for _, v := range slice {
+			out = append(out, telemetry.BoolValue(v))
+		}
+		return telemetry.SliceValue(out...)
+	case attribute.INT64SLICE:
+		slice := value.AsInt64Slice()
+		out := make([]telemetry.Value, 0, len(slice))
+		for _, v := range slice {
+			out = append(out, telemetry.Int64Value(v))
+		}
+		return telemetry.SliceValue(out...)
+	case attribute.FLOAT64SLICE:
+		slice := value.AsFloat64Slice()
+		out := make([]telemetry.Value, 0, len(slice))
+		for _, v := range slice {
+			out = append(out, telemetry.Float64Value(v))
+		}
+		return telemetry.SliceValue(out...)
+	case attribute.STRINGSLICE:
+		slice := value.AsStringSlice()
+		out := make([]telemetry.Value, 0, len(slice))
+		for _, v := range slice {
+			v = truncate(maxSpan.AttrValueLen, v)
+			out = append(out, telemetry.StringValue(v))
+		}
+		return telemetry.SliceValue(out...)
+	}
+	return telemetry.Value{}
+}
+
+// truncate returns a truncated version of s such that it contains less than
+// the limit number of characters. Truncation is applied by returning the limit
+// number of valid characters contained in s.
+//
+// If limit is negative, it returns the original string.
+//
+// UTF-8 is supported. When truncating, all invalid characters are dropped
+// before applying truncation.
+//
+// If s already contains less than the limit number of bytes, it is returned
+// unchanged. No invalid characters are removed.
+func truncate(limit int, s string) string {
+	// This prioritize performance in the following order based on the most
+	// common expected use-cases.
+	//
+	//  - Short values less than the default limit (128).
+	//  - Strings with valid encodings that exceed the limit.
+	//  - No limit.
+	//  - Strings with invalid encodings that exceed the limit.
+	if limit < 0 || len(s) <= limit {
+		return s
+	}
+
+	// Optimistically, assume all valid UTF-8.
+	var b strings.Builder
+	count := 0
+	for i, c := range s {
+		if c != utf8.RuneError {
+			count++
+			if count > limit {
+				return s[:i]
+			}
+			continue
+		}
+
+		_, size := utf8.DecodeRuneInString(s[i:])
+		if size == 1 {
+			// Invalid encoding.
+			b.Grow(len(s) - 1)
+			_, _ = b.WriteString(s[:i])
+			s = s[i:]
+			break
+		}
+	}
+
+	// Fast-path, no invalid input.
+	if b.Cap() == 0 {
+		return s
+	}
+
+	// Truncate while validating UTF-8.
+	for i := 0; i < len(s) && count < limit; {
+		c := s[i]
+		if c < utf8.RuneSelf {
+			// Optimization for single byte runes (common case).
+			_ = b.WriteByte(c)
+			i++
+			count++
+			continue
+		}
+
+		_, size := utf8.DecodeRuneInString(s[i:])
+		if size == 1 {
+			// We checked for all 1-byte runes above, this is a RuneError.
+			i++
+			continue
+		}
+
+		_, _ = b.WriteString(s[i : i+size])
+		i += size
+		count++
+	}
+
+	return b.String()
+}
+
+func (s *autoSpan) End(opts ...SpanEndOption) {
+	if s == nil || !s.sampled.Swap(false) {
+		return
+	}
+
+	// s.end exists so the lock (s.mu) is not held while s.ended is called.
+	s.ended(s.end(opts))
+}
+
+func (s *autoSpan) end(opts []SpanEndOption) []byte {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	cfg := NewSpanEndConfig(opts...)
+	if t := cfg.Timestamp(); !t.IsZero() {
+		s.span.EndTime = cfg.Timestamp()
+	} else {
+		s.span.EndTime = time.Now()
+	}
+
+	b, _ := json.Marshal(s.traces) // TODO: do not ignore this error.
+	return b
+}
+
+// Expected to be implemented in eBPF.
+//
+//go:noinline
+func (*autoSpan) ended(buf []byte) { ended(buf) }
+
+// ended is used for testing.
+var ended = func([]byte) {}
+
+func (s *autoSpan) RecordError(err error, opts ...EventOption) {
+	if s == nil || err == nil || !s.sampled.Load() {
+		return
+	}
+
+	cfg := NewEventConfig(opts...)
+
+	attrs := cfg.Attributes()
+	attrs = append(attrs,
+		semconv.ExceptionType(typeStr(err)),
+		semconv.ExceptionMessage(err.Error()),
+	)
+	if cfg.StackTrace() {
+		buf := make([]byte, 2048)
+		n := runtime.Stack(buf, false)
+		attrs = append(attrs, semconv.ExceptionStacktrace(string(buf[0:n])))
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.addEvent(semconv.ExceptionEventName, cfg.Timestamp(), attrs)
+}
+
+func typeStr(i any) string {
+	t := reflect.TypeOf(i)
+	if t.PkgPath() == "" && t.Name() == "" {
+		// Likely a builtin type.
+		return t.String()
+	}
+	return fmt.Sprintf("%s.%s", t.PkgPath(), t.Name())
+}
+
+func (s *autoSpan) AddEvent(name string, opts ...EventOption) {
+	if s == nil || !s.sampled.Load() {
+		return
+	}
+
+	cfg := NewEventConfig(opts...)
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.addEvent(name, cfg.Timestamp(), cfg.Attributes())
+}
+
+// addEvent adds an event with name and attrs at tStamp to the span. The span
+// lock (s.mu) needs to be held by the caller.
+func (s *autoSpan) addEvent(name string, tStamp time.Time, attrs []attribute.KeyValue) {
+	limit := maxSpan.Events
+
+	if limit == 0 {
+		s.span.DroppedEvents++
+		return
+	}
+
+	if limit > 0 && len(s.span.Events) == limit {
+		// Drop head while avoiding allocation of more capacity.
+		copy(s.span.Events[:limit-1], s.span.Events[1:])
+		s.span.Events = s.span.Events[:limit-1]
+		s.span.DroppedEvents++
+	}
+
+	e := &telemetry.SpanEvent{Time: tStamp, Name: name}
+	e.Attrs, e.DroppedAttrs = convCappedAttrs(maxSpan.EventAttrs, attrs)
+
+	s.span.Events = append(s.span.Events, e)
+}
+
+func (s *autoSpan) AddLink(link Link) {
+	if s == nil || !s.sampled.Load() {
+		return
+	}
+
+	l := maxSpan.Links
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if l == 0 {
+		s.span.DroppedLinks++
+		return
+	}
+
+	if l > 0 && len(s.span.Links) == l {
+		// Drop head while avoiding allocation of more capacity.
+		copy(s.span.Links[:l-1], s.span.Links[1:])
+		s.span.Links = s.span.Links[:l-1]
+		s.span.DroppedLinks++
+	}
+
+	s.span.Links = append(s.span.Links, convLink(link))
+}
+
+func convLinks(links []Link) []*telemetry.SpanLink {
+	out := make([]*telemetry.SpanLink, 0, len(links))
+	for _, link := range links {
+		out = append(out, convLink(link))
+	}
+	return out
+}
+
+func convLink(link Link) *telemetry.SpanLink {
+	l := &telemetry.SpanLink{
+		TraceID:    telemetry.TraceID(link.SpanContext.TraceID()),
+		SpanID:     telemetry.SpanID(link.SpanContext.SpanID()),
+		TraceState: link.SpanContext.TraceState().String(),
+		Flags:      uint32(link.SpanContext.TraceFlags()),
+	}
+	l.Attrs, l.DroppedAttrs = convCappedAttrs(maxSpan.LinkAttrs, link.Attributes)
+
+	return l
+}
+
+func (s *autoSpan) SetName(name string) {
+	if s == nil || !s.sampled.Load() {
+		return
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.span.Name = name
+}
+
+func (*autoSpan) TracerProvider() TracerProvider { return newAutoTracerProvider() }
+
+// maxSpan are the span limits resolved during startup.
+var maxSpan = newSpanLimits()
+
+type spanLimits struct {
+	// Attrs is the number of allowed attributes for a span.
+	//
+	// This is resolved from the environment variable value for the
+	// OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT key if it exists. Otherwise, the
+	// environment variable value for OTEL_ATTRIBUTE_COUNT_LIMIT, or 128 if
+	// that is not set, is used.
+	Attrs int
+	// AttrValueLen is the maximum attribute value length allowed for a span.
+	//
+	// This is resolved from the environment variable value for the
+	// OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT key if it exists. Otherwise, the
+	// environment variable value for OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT, or -1
+	// if that is not set, is used.
+	AttrValueLen int
+	// Events is the number of allowed events for a span.
+	//
+	// This is resolved from the environment variable value for the
+	// OTEL_SPAN_EVENT_COUNT_LIMIT key, or 128 is used if that is not set.
+	Events int
+	// EventAttrs is the number of allowed attributes for a span event.
+	//
+	// The is resolved from the environment variable value for the
+	// OTEL_EVENT_ATTRIBUTE_COUNT_LIMIT key, or 128 is used if that is not set.
+	EventAttrs int
+	// Links is the number of allowed Links for a span.
+	//
+	// This is resolved from the environment variable value for the
+	// OTEL_SPAN_LINK_COUNT_LIMIT, or 128 is used if that is not set.
+	Links int
+	// LinkAttrs is the number of allowed attributes for a span link.
+	//
+	// This is resolved from the environment variable value for the
+	// OTEL_LINK_ATTRIBUTE_COUNT_LIMIT, or 128 is used if that is not set.
+	LinkAttrs int
+}
+
+func newSpanLimits() spanLimits {
+	return spanLimits{
+		Attrs: firstEnv(
+			128,
+			"OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT",
+			"OTEL_ATTRIBUTE_COUNT_LIMIT",
+		),
+		AttrValueLen: firstEnv(
+			-1, // Unlimited.
+			"OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT",
+			"OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT",
+		),
+		Events:     firstEnv(128, "OTEL_SPAN_EVENT_COUNT_LIMIT"),
+		EventAttrs: firstEnv(128, "OTEL_EVENT_ATTRIBUTE_COUNT_LIMIT"),
+		Links:      firstEnv(128, "OTEL_SPAN_LINK_COUNT_LIMIT"),
+		LinkAttrs:  firstEnv(128, "OTEL_LINK_ATTRIBUTE_COUNT_LIMIT"),
+	}
+}
+
+// firstEnv returns the parsed integer value of the first matching environment
+// variable from keys. The defaultVal is returned if the value is not an
+// integer or no match is found.
+func firstEnv(defaultVal int, keys ...string) int {
+	for _, key := range keys {
+		strV := os.Getenv(key)
+		if strV == "" {
+			continue
+		}
+
+		v, err := strconv.Atoi(strV)
+		if err == nil {
+			return v
+		}
+		// Ignore invalid environment variable.
+	}
+
+	return defaultVal
+}

trace/internal/telemetry/attr.go

@@ -0,0 +1,58 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+// Attr is a key-value pair.
+type Attr struct {
+	Key   string `json:"key,omitempty"`
+	Value Value  `json:"value,omitempty"`
+}
+
+// String returns an Attr for a string value.
+func String(key, value string) Attr {
+	return Attr{key, StringValue(value)}
+}
+
+// Int64 returns an Attr for an int64 value.
+func Int64(key string, value int64) Attr {
+	return Attr{key, Int64Value(value)}
+}
+
+// Int returns an Attr for an int value.
+func Int(key string, value int) Attr {
+	return Int64(key, int64(value))
+}
+
+// Float64 returns an Attr for a float64 value.
+func Float64(key string, value float64) Attr {
+	return Attr{key, Float64Value(value)}
+}
+
+// Bool returns an Attr for a bool value.
+func Bool(key string, value bool) Attr {
+	return Attr{key, BoolValue(value)}
+}
+
+// Bytes returns an Attr for a []byte value.
+// The passed slice must not be changed after it is passed.
+func Bytes(key string, value []byte) Attr {
+	return Attr{key, BytesValue(value)}
+}
+
+// Slice returns an Attr for a []Value value.
+// The passed slice must not be changed after it is passed.
+func Slice(key string, value ...Value) Attr {
+	return Attr{key, SliceValue(value...)}
+}
+
+// Map returns an Attr for a map value.
+// The passed slice must not be changed after it is passed.
+func Map(key string, value ...Attr) Attr {
+	return Attr{key, MapValue(value...)}
+}
+
+// Equal returns if a is equal to b.
+func (a Attr) Equal(b Attr) bool {
+	return a.Key == b.Key && a.Value.Equal(b.Value)
+}

trace/internal/telemetry/attr_test.go

@@ -0,0 +1,156 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import "testing"
+
+func TestAttrEncoding(t *testing.T) {
+	attrs := []Attr{
+		String("user", "Alice"),
+		Bool("admin", true),
+		Int64("floor", -2),
+		Float64("impact", 0.21362),
+		Slice("reports", StringValue("Bob"), StringValue("Dave")),
+		Map("favorites", String("food", "hot dog"), Int("number", 13)),
+		Bytes("secret", []byte("NUI4RUZGRjc5ODAzODEwM0QyNjlCNjMzODEzRkM2MEM=")),
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(attrs, []byte(`[
+		{
+			"key": "user",
+			"value": {
+				"stringValue": "Alice"
+			}
+		},
+		{
+			"key": "admin",
+			"value": {
+				"boolValue": true
+			}
+		},
+		{
+			"key": "floor",
+			"value": {
+				"intValue": "-2"
+			}
+		},
+		{
+			"key": "impact",
+			"value": {
+				"doubleValue": 0.21362
+			}
+		},
+		{
+			"key": "reports",
+			"value": {
+				"arrayValue": {
+					"values": [
+						{
+							"stringValue": "Bob"
+						},
+						{
+							"stringValue": "Dave"
+						}
+					]
+				}
+			}
+		},
+		{
+			"key": "favorites",
+			"value": {
+				"kvlistValue": {
+					"values": [
+						{
+							"key": "food",
+							"value": {
+								"stringValue": "hot dog"
+							}
+						},
+						{
+							"key": "number",
+							"value": {
+								"intValue": "13"
+							}
+						}
+					]
+				}
+			}
+		},
+		{
+			"key": "secret",
+			"value": {
+				"bytesValue": "TlVJNFJVWkdSamM1T0RBek9ERXdNMFF5TmpsQ05qTXpPREV6UmtNMk1FTT0="
+			}
+		}
+	]`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(attrs, []byte(`[
+		{
+			"key": "user",
+			"value": {
+				"string_value": "Alice"
+			}
+		},
+		{
+			"key": "admin",
+			"value": {
+				"bool_value": true
+			}
+		},
+		{
+			"key": "floor",
+			"value": {
+				"int_value": "-2"
+			}
+		},
+		{
+			"key": "impact",
+			"value": {
+				"double_value": 0.21362
+			}
+		},
+		{
+			"key": "reports",
+			"value": {
+				"array_value": {
+					"values": [
+						{
+							"string_value": "Bob"
+						},
+						{
+							"string_value": "Dave"
+						}
+					]
+				}
+			}
+		},
+		{
+			"key": "favorites",
+			"value": {
+				"kvlist_value": {
+					"values": [
+						{
+							"key": "food",
+							"value": {
+								"string_value": "hot dog"
+							}
+						},
+						{
+							"key": "number",
+							"value": {
+								"int_value": "13"
+							}
+						}
+					]
+				}
+			}
+		},
+		{
+			"key": "secret",
+			"value": {
+				"bytes_value": "TlVJNFJVWkdSamM1T0RBek9ERXdNMFF5TmpsQ05qTXpPREV6UmtNMk1FTT0="
+			}
+		}
+	]`)))
+}

trace/internal/telemetry/bench_test.go

@@ -0,0 +1,70 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+	"time"
+)
+
+var (
+	traceID = [16]byte{0x1}
+
+	spanID1 = [8]byte{0x1}
+	spanID2 = [8]byte{0x2}
+
+	now      = time.Now()
+	nowPlus1 = now.Add(1 * time.Second)
+
+	spanA = &Span{
+		TraceID:      traceID,
+		SpanID:       spanID2,
+		ParentSpanID: spanID1,
+		Flags:        1,
+		Name:         "span-a",
+		StartTime:    now,
+		EndTime:      nowPlus1,
+		Status: &Status{
+			Message: "test status",
+			Code:    StatusCodeOK,
+		},
+	}
+
+	spanB = &Span{}
+
+	scopeSpans = &ScopeSpans{
+		Scope: &Scope{
+			Name:    "TestTracer",
+			Version: "v0.1.0",
+		},
+		SchemaURL: "http://go.opentelemetry.io/test",
+		Spans:     []*Span{spanA, spanB},
+	}
+)
+
+func BenchmarkJSONMarshalUnmarshal(b *testing.B) {
+	var out ScopeSpans
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		var inBuf bytes.Buffer
+		enc := json.NewEncoder(&inBuf)
+		err := enc.Encode(scopeSpans)
+		if err != nil {
+			b.Fatal(err)
+		}
+
+		payload := inBuf.Bytes()
+
+		dec := json.NewDecoder(bytes.NewReader(payload))
+		err = dec.Decode(&out)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+	_ = out
+}

trace/internal/telemetry/conv_test.go

@@ -0,0 +1,50 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const schema100 = "http://go.opentelemetry.io/schema/v1.0.0"
+
+var y2k = time.Unix(0, time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC).UnixNano()) // No location.
+
+func runJSONEncodingTests[T any](decoded T, encoded []byte) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+
+		t.Run("Unmarshal", runJSONUnmarshalTest(decoded, encoded))
+		t.Run("Marshal", runJSONMarshalTest(decoded, encoded))
+	}
+}
+
+func runJSONMarshalTest[T any](decoded T, encoded []byte) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+
+		got, err := json.Marshal(decoded)
+		require.NoError(t, err)
+
+		var want bytes.Buffer
+		require.NoError(t, json.Compact(&want, encoded))
+		assert.Equal(t, want.String(), string(got))
+	}
+}
+
+func runJSONUnmarshalTest[T any](decoded T, encoded []byte) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+
+		var got T
+		require.NoError(t, json.Unmarshal(encoded, &got))
+		assert.Equal(t, decoded, got)
+	}
+}

trace/internal/telemetry/doc.go

@@ -0,0 +1,8 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+/*
+Package telemetry provides a lightweight representations of OpenTelemetry
+telemetry that is compatible with the OTLP JSON protobuf encoding.
+*/
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"

trace/internal/telemetry/id.go

@@ -0,0 +1,103 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"encoding/hex"
+	"errors"
+	"fmt"
+)
+
+const (
+	traceIDSize = 16
+	spanIDSize  = 8
+)
+
+// TraceID is a custom data type that is used for all trace IDs.
+type TraceID [traceIDSize]byte
+
+// String returns the hex string representation form of a TraceID.
+func (tid TraceID) String() string {
+	return hex.EncodeToString(tid[:])
+}
+
+// IsEmpty returns false if id contains at least one non-zero byte.
+func (tid TraceID) IsEmpty() bool {
+	return tid == [traceIDSize]byte{}
+}
+
+// MarshalJSON converts the trace ID into a hex string enclosed in quotes.
+func (tid TraceID) MarshalJSON() ([]byte, error) {
+	if tid.IsEmpty() {
+		return []byte(`""`), nil
+	}
+	return marshalJSON(tid[:])
+}
+
+// UnmarshalJSON inflates the trace ID from hex string, possibly enclosed in
+// quotes.
+func (tid *TraceID) UnmarshalJSON(data []byte) error {
+	*tid = [traceIDSize]byte{}
+	return unmarshalJSON(tid[:], data)
+}
+
+// SpanID is a custom data type that is used for all span IDs.
+type SpanID [spanIDSize]byte
+
+// String returns the hex string representation form of a SpanID.
+func (sid SpanID) String() string {
+	return hex.EncodeToString(sid[:])
+}
+
+// IsEmpty returns true if the span ID contains at least one non-zero byte.
+func (sid SpanID) IsEmpty() bool {
+	return sid == [spanIDSize]byte{}
+}
+
+// MarshalJSON converts span ID into a hex string enclosed in quotes.
+func (sid SpanID) MarshalJSON() ([]byte, error) {
+	if sid.IsEmpty() {
+		return []byte(`""`), nil
+	}
+	return marshalJSON(sid[:])
+}
+
+// UnmarshalJSON decodes span ID from hex string, possibly enclosed in quotes.
+func (sid *SpanID) UnmarshalJSON(data []byte) error {
+	*sid = [spanIDSize]byte{}
+	return unmarshalJSON(sid[:], data)
+}
+
+// marshalJSON converts id into a hex string enclosed in quotes.
+func marshalJSON(id []byte) ([]byte, error) {
+	// Plus 2 quote chars at the start and end.
+	hexLen := hex.EncodedLen(len(id)) + 2
+
+	b := make([]byte, hexLen)
+	hex.Encode(b[1:hexLen-1], id)
+	b[0], b[hexLen-1] = '"', '"'
+
+	return b, nil
+}
+
+// unmarshalJSON inflates trace id from hex string, possibly enclosed in quotes.
+func unmarshalJSON(dst []byte, src []byte) error {
+	if l := len(src); l >= 2 && src[0] == '"' && src[l-1] == '"' {
+		src = src[1 : l-1]
+	}
+	nLen := len(src)
+	if nLen == 0 {
+		return nil
+	}
+
+	if len(dst) != hex.DecodedLen(nLen) {
+		return errors.New("invalid length for ID")
+	}
+
+	_, err := hex.Decode(dst, src)
+	if err != nil {
+		return fmt.Errorf("cannot unmarshal ID from string '%s': %w", string(src), err)
+	}
+	return nil
+}

trace/internal/telemetry/number.go

@@ -0,0 +1,67 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"encoding/json"
+	"strconv"
+)
+
+// protoInt64 represents the protobuf encoding of integers which can be either
+// strings or integers.
+type protoInt64 int64
+
+// Int64 returns the protoInt64 as an int64.
+func (i *protoInt64) Int64() int64 { return int64(*i) }
+
+// UnmarshalJSON decodes both strings and integers.
+func (i *protoInt64) UnmarshalJSON(data []byte) error {
+	if data[0] == '"' {
+		var str string
+		if err := json.Unmarshal(data, &str); err != nil {
+			return err
+		}
+		parsedInt, err := strconv.ParseInt(str, 10, 64)
+		if err != nil {
+			return err
+		}
+		*i = protoInt64(parsedInt)
+	} else {
+		var parsedInt int64
+		if err := json.Unmarshal(data, &parsedInt); err != nil {
+			return err
+		}
+		*i = protoInt64(parsedInt)
+	}
+	return nil
+}
+
+// protoUint64 represents the protobuf encoding of integers which can be either
+// strings or integers.
+type protoUint64 uint64
+
+// Int64 returns the protoUint64 as a uint64.
+func (i *protoUint64) Uint64() uint64 { return uint64(*i) }
+
+// UnmarshalJSON decodes both strings and integers.
+func (i *protoUint64) UnmarshalJSON(data []byte) error {
+	if data[0] == '"' {
+		var str string
+		if err := json.Unmarshal(data, &str); err != nil {
+			return err
+		}
+		parsedUint, err := strconv.ParseUint(str, 10, 64)
+		if err != nil {
+			return err
+		}
+		*i = protoUint64(parsedUint)
+	} else {
+		var parsedUint uint64
+		if err := json.Unmarshal(data, &parsedUint); err != nil {
+			return err
+		}
+		*i = protoUint64(parsedUint)
+	}
+	return nil
+}

trace/internal/telemetry/resource.go

@@ -0,0 +1,66 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// Resource information.
+type Resource struct {
+	// Attrs are the set of attributes that describe the resource. Attribute
+	// keys MUST be unique (it is not allowed to have more than one attribute
+	// with the same key).
+	Attrs []Attr `json:"attributes,omitempty"`
+	// DroppedAttrs is the number of dropped attributes. If the value
+	// is 0, then no attributes were dropped.
+	DroppedAttrs uint32 `json:"droppedAttributesCount,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into r.
+func (r *Resource) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid Resource type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid Resource field: %#v", keyIface)
+		}
+
+		switch key {
+		case "attributes":
+			err = decoder.Decode(&r.Attrs)
+		case "droppedAttributesCount", "dropped_attributes_count":
+			err = decoder.Decode(&r.DroppedAttrs)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}

trace/internal/telemetry/resource_test.go

@@ -0,0 +1,37 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import "testing"
+
+func TestResourceEncoding(t *testing.T) {
+	res := &Resource{
+		Attrs:        []Attr{String("key", "val")},
+		DroppedAttrs: 10,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(res, []byte(`{
+		"attributes": [
+			{
+				"key": "key",
+				"value": {
+					"stringValue": "val"
+				}
+			}
+		],
+		"droppedAttributesCount": 10
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(res, []byte(`{
+		"attributes": [
+			{
+				"key": "key",
+				"value": {
+					"string_value": "val"
+				}
+			}
+		],
+		"dropped_attributes_count": 10
+	}`)))
+}

trace/internal/telemetry/scope.go

@@ -0,0 +1,67 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// Scope is the identifying values of the instrumentation scope.
+type Scope struct {
+	Name         string `json:"name,omitempty"`
+	Version      string `json:"version,omitempty"`
+	Attrs        []Attr `json:"attributes,omitempty"`
+	DroppedAttrs uint32 `json:"droppedAttributesCount,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into r.
+func (s *Scope) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid Scope type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid Scope field: %#v", keyIface)
+		}
+
+		switch key {
+		case "name":
+			err = decoder.Decode(&s.Name)
+		case "version":
+			err = decoder.Decode(&s.Version)
+		case "attributes":
+			err = decoder.Decode(&s.Attrs)
+		case "droppedAttributesCount", "dropped_attributes_count":
+			err = decoder.Decode(&s.DroppedAttrs)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}

trace/internal/telemetry/scope_test.go

@@ -0,0 +1,43 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import "testing"
+
+func TestScopeEncoding(t *testing.T) {
+	scope := &Scope{
+		Name:         "go.opentelemetry.io/otel/trace/internal/telemetry/test",
+		Version:      "v0.0.1",
+		Attrs:        []Attr{String("department", "ops")},
+		DroppedAttrs: 1,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(scope, []byte(`{
+		"name": "go.opentelemetry.io/otel/trace/internal/telemetry/test",
+		"version": "v0.0.1",
+		"attributes": [
+			{
+				"key": "department",
+				"value": {
+					"stringValue": "ops"
+				}
+			}
+		],
+		"droppedAttributesCount": 1
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(scope, []byte(`{
+		"name": "go.opentelemetry.io/otel/trace/internal/telemetry/test",
+		"version": "v0.0.1",
+		"attributes": [
+			{
+				"key": "department",
+				"value": {
+					"string_value": "ops"
+				}
+			}
+		],
+		"dropped_attributes_count": 1
+	}`)))
+}

trace/internal/telemetry/span.go

@@ -0,0 +1,460 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"bytes"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"math"
+	"time"
+)
+
+// A Span represents a single operation performed by a single component of the
+// system.
+type Span struct {
+	// A unique identifier for a trace. All spans from the same trace share
+	// the same `trace_id`. The ID is a 16-byte array. An ID with all zeroes OR
+	// of length other than 16 bytes is considered invalid (empty string in OTLP/JSON
+	// is zero-length and thus is also invalid).
+	//
+	// This field is required.
+	TraceID TraceID `json:"traceId,omitempty"`
+	// A unique identifier for a span within a trace, assigned when the span
+	// is created. The ID is an 8-byte array. An ID with all zeroes OR of length
+	// other than 8 bytes is considered invalid (empty string in OTLP/JSON
+	// is zero-length and thus is also invalid).
+	//
+	// This field is required.
+	SpanID SpanID `json:"spanId,omitempty"`
+	// trace_state conveys information about request position in multiple distributed tracing graphs.
+	// It is a trace_state in w3c-trace-context format: https://www.w3.org/TR/trace-context/#tracestate-header
+	// See also https://github.com/w3c/distributed-tracing for more details about this field.
+	TraceState string `json:"traceState,omitempty"`
+	// The `span_id` of this span's parent span. If this is a root span, then this
+	// field must be empty. The ID is an 8-byte array.
+	ParentSpanID SpanID `json:"parentSpanId,omitempty"`
+	// Flags, a bit field.
+	//
+	// Bits 0-7 (8 least significant bits) are the trace flags as defined in W3C Trace
+	// Context specification. To read the 8-bit W3C trace flag, use
+	// `flags & SPAN_FLAGS_TRACE_FLAGS_MASK`.
+	//
+	// See https://www.w3.org/TR/trace-context-2/#trace-flags for the flag definitions.
+	//
+	// Bits 8 and 9 represent the 3 states of whether a span's parent
+	// is remote. The states are (unknown, is not remote, is remote).
+	// To read whether the value is known, use `(flags & SPAN_FLAGS_CONTEXT_HAS_IS_REMOTE_MASK) != 0`.
+	// To read whether the span is remote, use `(flags & SPAN_FLAGS_CONTEXT_IS_REMOTE_MASK) != 0`.
+	//
+	// When creating span messages, if the message is logically forwarded from another source
+	// with an equivalent flags fields (i.e., usually another OTLP span message), the field SHOULD
+	// be copied as-is. If creating from a source that does not have an equivalent flags field
+	// (such as a runtime representation of an OpenTelemetry span), the high 22 bits MUST
+	// be set to zero.
+	// Readers MUST NOT assume that bits 10-31 (22 most significant bits) will be zero.
+	//
+	// [Optional].
+	Flags uint32 `json:"flags,omitempty"`
+	// A description of the span's operation.
+	//
+	// For example, the name can be a qualified method name or a file name
+	// and a line number where the operation is called. A best practice is to use
+	// the same display name at the same call point in an application.
+	// This makes it easier to correlate spans in different traces.
+	//
+	// This field is semantically required to be set to non-empty string.
+	// Empty value is equivalent to an unknown span name.
+	//
+	// This field is required.
+	Name string `json:"name"`
+	// Distinguishes between spans generated in a particular context. For example,
+	// two spans with the same name may be distinguished using `CLIENT` (caller)
+	// and `SERVER` (callee) to identify queueing latency associated with the span.
+	Kind SpanKind `json:"kind,omitempty"`
+	// start_time_unix_nano is the start time of the span. On the client side, this is the time
+	// kept by the local machine where the span execution starts. On the server side, this
+	// is the time when the server's application handler starts running.
+	// Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January 1970.
+	//
+	// This field is semantically required and it is expected that end_time >= start_time.
+	StartTime time.Time `json:"startTimeUnixNano,omitempty"`
+	// end_time_unix_nano is the end time of the span. On the client side, this is the time
+	// kept by the local machine where the span execution ends. On the server side, this
+	// is the time when the server application handler stops running.
+	// Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January 1970.
+	//
+	// This field is semantically required and it is expected that end_time >= start_time.
+	EndTime time.Time `json:"endTimeUnixNano,omitempty"`
+	// attributes is a collection of key/value pairs. Note, global attributes
+	// like server name can be set using the resource API. Examples of attributes:
+	//
+	//     "/http/user_agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
+	//     "/http/server_latency": 300
+	//     "example.com/myattribute": true
+	//     "example.com/score": 10.239
+	//
+	// The OpenTelemetry API specification further restricts the allowed value types:
+	// https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute
+	// Attribute keys MUST be unique (it is not allowed to have more than one
+	// attribute with the same key).
+	Attrs []Attr `json:"attributes,omitempty"`
+	// dropped_attributes_count is the number of attributes that were discarded. Attributes
+	// can be discarded because their keys are too long or because there are too many
+	// attributes. If this value is 0, then no attributes were dropped.
+	DroppedAttrs uint32 `json:"droppedAttributesCount,omitempty"`
+	// events is a collection of Event items.
+	Events []*SpanEvent `json:"events,omitempty"`
+	// dropped_events_count is the number of dropped events. If the value is 0, then no
+	// events were dropped.
+	DroppedEvents uint32 `json:"droppedEventsCount,omitempty"`
+	// links is a collection of Links, which are references from this span to a span
+	// in the same or different trace.
+	Links []*SpanLink `json:"links,omitempty"`
+	// dropped_links_count is the number of dropped links after the maximum size was
+	// enforced. If this value is 0, then no links were dropped.
+	DroppedLinks uint32 `json:"droppedLinksCount,omitempty"`
+	// An optional final status for this span. Semantically when Status isn't set, it means
+	// span's status code is unset, i.e. assume STATUS_CODE_UNSET (code = 0).
+	Status *Status `json:"status,omitempty"`
+}
+
+// MarshalJSON encodes s into OTLP formatted JSON.
+func (s Span) MarshalJSON() ([]byte, error) {
+	startT := s.StartTime.UnixNano()
+	if s.StartTime.IsZero() || startT < 0 {
+		startT = 0
+	}
+
+	endT := s.EndTime.UnixNano()
+	if s.EndTime.IsZero() || endT < 0 {
+		endT = 0
+	}
+
+	// Override non-empty default SpanID marshal and omitempty.
+	var parentSpanId string
+	if !s.ParentSpanID.IsEmpty() {
+		b := make([]byte, hex.EncodedLen(spanIDSize))
+		hex.Encode(b, s.ParentSpanID[:])
+		parentSpanId = string(b)
+	}
+
+	type Alias Span
+	return json.Marshal(struct {
+		Alias
+		ParentSpanID string `json:"parentSpanId,omitempty"`
+		StartTime    uint64 `json:"startTimeUnixNano,omitempty"`
+		EndTime      uint64 `json:"endTimeUnixNano,omitempty"`
+	}{
+		Alias:        Alias(s),
+		ParentSpanID: parentSpanId,
+		StartTime:    uint64(startT), // nolint:gosec  // >0 checked above.
+		EndTime:      uint64(endT),   // nolint:gosec  // >0 checked above.
+	})
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into s.
+func (s *Span) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid Span type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid Span field: %#v", keyIface)
+		}
+
+		switch key {
+		case "traceId", "trace_id":
+			err = decoder.Decode(&s.TraceID)
+		case "spanId", "span_id":
+			err = decoder.Decode(&s.SpanID)
+		case "traceState", "trace_state":
+			err = decoder.Decode(&s.TraceState)
+		case "parentSpanId", "parent_span_id":
+			err = decoder.Decode(&s.ParentSpanID)
+		case "flags":
+			err = decoder.Decode(&s.Flags)
+		case "name":
+			err = decoder.Decode(&s.Name)
+		case "kind":
+			err = decoder.Decode(&s.Kind)
+		case "startTimeUnixNano", "start_time_unix_nano":
+			var val protoUint64
+			err = decoder.Decode(&val)
+			v := int64(min(val.Uint64(), math.MaxInt64)) // nolint: gosec  // Overflow checked.
+			s.StartTime = time.Unix(0, v)
+		case "endTimeUnixNano", "end_time_unix_nano":
+			var val protoUint64
+			err = decoder.Decode(&val)
+			v := int64(min(val.Uint64(), math.MaxInt64)) // nolint: gosec  // Overflow checked.
+			s.EndTime = time.Unix(0, v)
+		case "attributes":
+			err = decoder.Decode(&s.Attrs)
+		case "droppedAttributesCount", "dropped_attributes_count":
+			err = decoder.Decode(&s.DroppedAttrs)
+		case "events":
+			err = decoder.Decode(&s.Events)
+		case "droppedEventsCount", "dropped_events_count":
+			err = decoder.Decode(&s.DroppedEvents)
+		case "links":
+			err = decoder.Decode(&s.Links)
+		case "droppedLinksCount", "dropped_links_count":
+			err = decoder.Decode(&s.DroppedLinks)
+		case "status":
+			err = decoder.Decode(&s.Status)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// SpanFlags represents constants used to interpret the
+// Span.flags field, which is protobuf 'fixed32' type and is to
+// be used as bit-fields. Each non-zero value defined in this enum is
+// a bit-mask.  To extract the bit-field, for example, use an
+// expression like:
+//
+//	(span.flags & SPAN_FLAGS_TRACE_FLAGS_MASK)
+//
+// See https://www.w3.org/TR/trace-context-2/#trace-flags for the flag definitions.
+//
+// Note that Span flags were introduced in version 1.1 of the
+// OpenTelemetry protocol.  Older Span producers do not set this
+// field, consequently consumers should not rely on the absence of a
+// particular flag bit to indicate the presence of a particular feature.
+type SpanFlags int32
+
+const (
+	// Bits 0-7 are used for trace flags.
+	SpanFlagsTraceFlagsMask SpanFlags = 255
+	// Bits 8 and 9 are used to indicate that the parent span or link span is remote.
+	// Bit 8 (`HAS_IS_REMOTE`) indicates whether the value is known.
+	// Bit 9 (`IS_REMOTE`) indicates whether the span or link is remote.
+	SpanFlagsContextHasIsRemoteMask SpanFlags = 256
+	// SpanFlagsContextHasIsRemoteMask indicates the Span is remote.
+	SpanFlagsContextIsRemoteMask SpanFlags = 512
+)
+
+// SpanKind is the type of span. Can be used to specify additional relationships between spans
+// in addition to a parent/child relationship.
+type SpanKind int32
+
+const (
+	// Indicates that the span represents an internal operation within an application,
+	// as opposed to an operation happening at the boundaries. Default value.
+	SpanKindInternal SpanKind = 1
+	// Indicates that the span covers server-side handling of an RPC or other
+	// remote network request.
+	SpanKindServer SpanKind = 2
+	// Indicates that the span describes a request to some remote service.
+	SpanKindClient SpanKind = 3
+	// Indicates that the span describes a producer sending a message to a broker.
+	// Unlike CLIENT and SERVER, there is often no direct critical path latency relationship
+	// between producer and consumer spans. A PRODUCER span ends when the message was accepted
+	// by the broker while the logical processing of the message might span a much longer time.
+	SpanKindProducer SpanKind = 4
+	// Indicates that the span describes consumer receiving a message from a broker.
+	// Like the PRODUCER kind, there is often no direct critical path latency relationship
+	// between producer and consumer spans.
+	SpanKindConsumer SpanKind = 5
+)
+
+// Event is a time-stamped annotation of the span, consisting of user-supplied
+// text description and key-value pairs.
+type SpanEvent struct {
+	// time_unix_nano is the time the event occurred.
+	Time time.Time `json:"timeUnixNano,omitempty"`
+	// name of the event.
+	// This field is semantically required to be set to non-empty string.
+	Name string `json:"name,omitempty"`
+	// attributes is a collection of attribute key/value pairs on the event.
+	// Attribute keys MUST be unique (it is not allowed to have more than one
+	// attribute with the same key).
+	Attrs []Attr `json:"attributes,omitempty"`
+	// dropped_attributes_count is the number of dropped attributes. If the value is 0,
+	// then no attributes were dropped.
+	DroppedAttrs uint32 `json:"droppedAttributesCount,omitempty"`
+}
+
+// MarshalJSON encodes e into OTLP formatted JSON.
+func (e SpanEvent) MarshalJSON() ([]byte, error) {
+	t := e.Time.UnixNano()
+	if e.Time.IsZero() || t < 0 {
+		t = 0
+	}
+
+	type Alias SpanEvent
+	return json.Marshal(struct {
+		Alias
+		Time uint64 `json:"timeUnixNano,omitempty"`
+	}{
+		Alias: Alias(e),
+		Time:  uint64(t), // nolint: gosec  // >0 checked above
+	})
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into se.
+func (se *SpanEvent) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid SpanEvent type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid SpanEvent field: %#v", keyIface)
+		}
+
+		switch key {
+		case "timeUnixNano", "time_unix_nano":
+			var val protoUint64
+			err = decoder.Decode(&val)
+			v := int64(min(val.Uint64(), math.MaxInt64)) // nolint: gosec  // Overflow checked.
+			se.Time = time.Unix(0, v)
+		case "name":
+			err = decoder.Decode(&se.Name)
+		case "attributes":
+			err = decoder.Decode(&se.Attrs)
+		case "droppedAttributesCount", "dropped_attributes_count":
+			err = decoder.Decode(&se.DroppedAttrs)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// A pointer from the current span to another span in the same trace or in a
+// different trace. For example, this can be used in batching operations,
+// where a single batch handler processes multiple requests from different
+// traces or when the handler receives a request from a different project.
+type SpanLink struct {
+	// A unique identifier of a trace that this linked span is part of. The ID is a
+	// 16-byte array.
+	TraceID TraceID `json:"traceId,omitempty"`
+	// A unique identifier for the linked span. The ID is an 8-byte array.
+	SpanID SpanID `json:"spanId,omitempty"`
+	// The trace_state associated with the link.
+	TraceState string `json:"traceState,omitempty"`
+	// attributes is a collection of attribute key/value pairs on the link.
+	// Attribute keys MUST be unique (it is not allowed to have more than one
+	// attribute with the same key).
+	Attrs []Attr `json:"attributes,omitempty"`
+	// dropped_attributes_count is the number of dropped attributes. If the value is 0,
+	// then no attributes were dropped.
+	DroppedAttrs uint32 `json:"droppedAttributesCount,omitempty"`
+	// Flags, a bit field.
+	//
+	// Bits 0-7 (8 least significant bits) are the trace flags as defined in W3C Trace
+	// Context specification. To read the 8-bit W3C trace flag, use
+	// `flags & SPAN_FLAGS_TRACE_FLAGS_MASK`.
+	//
+	// See https://www.w3.org/TR/trace-context-2/#trace-flags for the flag definitions.
+	//
+	// Bits 8 and 9 represent the 3 states of whether the link is remote.
+	// The states are (unknown, is not remote, is remote).
+	// To read whether the value is known, use `(flags & SPAN_FLAGS_CONTEXT_HAS_IS_REMOTE_MASK) != 0`.
+	// To read whether the link is remote, use `(flags & SPAN_FLAGS_CONTEXT_IS_REMOTE_MASK) != 0`.
+	//
+	// Readers MUST NOT assume that bits 10-31 (22 most significant bits) will be zero.
+	// When creating new spans, bits 10-31 (most-significant 22-bits) MUST be zero.
+	//
+	// [Optional].
+	Flags uint32 `json:"flags,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into sl.
+func (sl *SpanLink) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid SpanLink type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid SpanLink field: %#v", keyIface)
+		}
+
+		switch key {
+		case "traceId", "trace_id":
+			err = decoder.Decode(&sl.TraceID)
+		case "spanId", "span_id":
+			err = decoder.Decode(&sl.SpanID)
+		case "traceState", "trace_state":
+			err = decoder.Decode(&sl.TraceState)
+		case "attributes":
+			err = decoder.Decode(&sl.Attrs)
+		case "droppedAttributesCount", "dropped_attributes_count":
+			err = decoder.Decode(&sl.DroppedAttrs)
+		case "flags":
+			err = decoder.Decode(&sl.Flags)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}

trace/internal/telemetry/span_test.go

@@ -0,0 +1,200 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSpanEncoding(t *testing.T) {
+	span := &Span{
+		TraceID:      [16]byte{0x1},
+		SpanID:       [8]byte{0x2},
+		TraceState:   "test=a",
+		ParentSpanID: [8]byte{0x1},
+		Flags:        1,
+		Name:         "span.a",
+		Kind:         SpanKindClient,
+		StartTime:    y2k,
+		EndTime:      y2k.Add(time.Second),
+		Attrs:        []Attr{String("key", "val")},
+		DroppedAttrs: 2,
+		Events: []*SpanEvent{{
+			Name: "name",
+		}},
+		DroppedEvents: 3,
+		Links: []*SpanLink{{
+			TraceID: TraceID{0x2},
+			SpanID:  SpanID{0x1},
+		}},
+		DroppedLinks: 4,
+		Status: &Status{
+			Message: "okay",
+			Code:    StatusCodeOK,
+		},
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(span, []byte(`{
+		"traceId": "01000000000000000000000000000000",
+		"spanId": "0200000000000000",
+		"traceState": "test=a",
+		"flags": 1,
+		"name": "span.a",
+		"kind": 3,
+		"attributes": [
+			{
+				"key": "key",
+				"value": {
+					"stringValue": "val"
+				}
+			}
+		],
+		"droppedAttributesCount": 2,
+		"events": [
+			{
+				"name": "name"
+			}
+		],
+		"droppedEventsCount": 3,
+		"links": [
+			{
+				"traceId": "02000000000000000000000000000000",
+				"spanId": "0100000000000000"
+			}
+		],
+		"droppedLinksCount": 4,
+		"status": {
+			"message": "okay",
+			"code": 1
+		},
+		"parentSpanId": "0100000000000000",
+		"startTimeUnixNano": 946684800000000000,
+		"endTimeUnixNano": 946684801000000000
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(span, []byte(`{
+		"trace_id": "01000000000000000000000000000000",
+		"span_id": "0200000000000000",
+		"trace_state": "test=a",
+		"flags": 1,
+		"name": "span.a",
+		"kind": 3,
+		"attributes": [
+			{
+				"key": "key",
+				"value": {
+					"string_value": "val"
+				}
+			}
+		],
+		"dropped_attributes_count": 2,
+		"events": [
+			{
+				"name": "name"
+			}
+		],
+		"dropped_events_count": 3,
+		"links": [
+			{
+				"trace_id": "02000000000000000000000000000000",
+				"span_id": "0100000000000000"
+			}
+		],
+		"dropped_links_count": 4,
+		"status": {
+			"message": "okay",
+			"code": 1
+		},
+		"parent_span_id": "0100000000000000",
+		"start_time_unix_nano": 946684800000000000,
+		"end_time_unix_nano": 946684801000000000
+	}`)))
+
+	t.Run("RequiredFields", runJSONMarshalTest(new(Span), []byte(`{
+		"traceId": "",
+		"spanId": "",
+		"name": ""
+	}`)))
+}
+
+func TestSpanEventEncoding(t *testing.T) {
+	event := &SpanEvent{
+		Time:         y2k.Add(10 * time.Microsecond),
+		Name:         "span.event",
+		Attrs:        []Attr{Float64("impact", 0.4372)},
+		DroppedAttrs: 2,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(event, []byte(`{
+		"name": "span.event",
+		"attributes": [
+			{
+				"key": "impact",
+				"value": {
+					"doubleValue": 0.4372
+				}
+			}
+		],
+		"droppedAttributesCount": 2,
+		"timeUnixNano": 946684800000010000
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(event, []byte(`{
+		"name": "span.event",
+		"attributes": [
+			{
+				"key": "impact",
+				"value": {
+					"double_value": 0.4372
+				}
+			}
+		],
+		"dropped_attributes_count": 2,
+		"time_unix_nano": 946684800000010000
+	}`)))
+}
+
+func TestSpanLinkEncoding(t *testing.T) {
+	link := &SpanLink{
+		TraceID:      TraceID{0x2},
+		SpanID:       SpanID{0x1},
+		TraceState:   "test=green",
+		Attrs:        []Attr{Int("queue", 17)},
+		DroppedAttrs: 8,
+		Flags:        1,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(link, []byte(`{
+		"traceId": "02000000000000000000000000000000",
+		"spanId": "0100000000000000",
+		"traceState": "test=green",
+		"attributes": [
+			{
+				"key": "queue",
+				"value": {
+					"intValue": "17"
+				}
+			}
+		],
+		"droppedAttributesCount": 8,
+		"flags": 1
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(link, []byte(`{
+		"trace_id": "02000000000000000000000000000000",
+		"span_id": "0100000000000000",
+		"trace_state": "test=green",
+		"attributes": [
+			{
+				"key": "queue",
+				"value": {
+					"int_value": "17"
+				}
+			}
+		],
+		"dropped_attributes_count": 8,
+		"flags": 1
+	}`)))
+}

trace/internal/telemetry/status.go

@@ -0,0 +1,40 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+// For the semantics of status codes see
+// https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-status
+type StatusCode int32
+
+const (
+	// The default status.
+	StatusCodeUnset StatusCode = 0
+	// The Span has been validated by an Application developer or Operator to
+	// have completed successfully.
+	StatusCodeOK StatusCode = 1
+	// The Span contains an error.
+	StatusCodeError StatusCode = 2
+)
+
+var statusCodeStrings = []string{
+	"Unset",
+	"OK",
+	"Error",
+}
+
+func (s StatusCode) String() string {
+	if s >= 0 && int(s) < len(statusCodeStrings) {
+		return statusCodeStrings[s]
+	}
+	return "<unknown telemetry.StatusCode>"
+}
+
+// The Status type defines a logical error model that is suitable for different
+// programming environments, including REST APIs and RPC APIs.
+type Status struct {
+	// A developer-facing human readable error message.
+	Message string `json:"message,omitempty"`
+	// The status code.
+	Code StatusCode `json:"code,omitempty"`
+}

trace/internal/telemetry/test/conversion_test.go

@@ -0,0 +1,244 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package test
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"go.opentelemetry.io/collector/pdata/pcommon"
+	"go.opentelemetry.io/collector/pdata/ptrace"
+
+	"go.opentelemetry.io/otel/trace/internal/telemetry"
+)
+
+var (
+	y2k = time.Unix(0, time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC).UnixNano()) // No location.
+
+	attrsA = []telemetry.Attr{
+		telemetry.String("user", "Alice"),
+		telemetry.Bool("admin", true),
+		telemetry.Int64("floor", -2),
+		telemetry.Float64("impact", 0.21362),
+		telemetry.Slice(
+			"reports",
+			telemetry.StringValue("Bob"),
+			telemetry.StringValue("Dave"),
+		),
+		telemetry.Map(
+			"favorites",
+			telemetry.String("food", "hot dog"),
+			telemetry.Int("number", 13),
+		),
+		telemetry.Bytes(
+			"secret",
+			[]byte("NUI4RUZGRjc5ODAzODEwM0QyNjlCNjMzODEzRkM2MEM="),
+		),
+	}
+	pAttrsA = func() pcommon.Map {
+		m := pcommon.NewMap()
+		m.PutStr("user", "Alice")
+		m.PutBool("admin", true)
+		m.PutInt("floor", -2)
+		m.PutDouble("impact", 0.21362)
+
+		s := m.PutEmptySlice("reports")
+		s.AppendEmpty().SetStr("Bob")
+		s.AppendEmpty().SetStr("Dave")
+
+		fav := m.PutEmptyMap("favorites")
+		fav.PutStr("food", "hot dog")
+		fav.PutInt("number", 13)
+
+		sec := m.PutEmptyBytes("secret")
+		sec.FromRaw([]byte("NUI4RUZGRjc5ODAzODEwM0QyNjlCNjMzODEzRkM2MEM="))
+
+		return m
+	}()
+
+	link = &telemetry.SpanLink{
+		TraceID:      telemetry.TraceID{0x2},
+		SpanID:       telemetry.SpanID{0x1},
+		TraceState:   "test=green",
+		Attrs:        []telemetry.Attr{telemetry.Int("queue", 17)},
+		DroppedAttrs: 8,
+		Flags:        1,
+	}
+	pLink = func() ptrace.SpanLink {
+		l := ptrace.NewSpanLink()
+		l.SetTraceID(pcommon.TraceID{0x2})
+		l.SetSpanID(pcommon.SpanID{0x1})
+		l.TraceState().FromRaw("test=green")
+		l.Attributes().PutInt("queue", 17)
+		l.SetDroppedAttributesCount(8)
+		l.SetFlags(1)
+		return l
+	}()
+
+	event = &telemetry.SpanEvent{
+		Time:         y2k.Add(10 * time.Microsecond),
+		Name:         "span.event",
+		Attrs:        []telemetry.Attr{telemetry.Float64("impact", 0.4372)},
+		DroppedAttrs: 2,
+	}
+	prevent = func() ptrace.SpanEvent {
+		e := ptrace.NewSpanEvent()
+		e.SetTimestamp(pcommon.NewTimestampFromTime(y2k.Add(10 * time.Microsecond)))
+		e.SetName("span.event")
+		e.Attributes().PutDouble("impact", 0.4372)
+		e.SetDroppedAttributesCount(2)
+		return e
+	}()
+
+	spanA = &telemetry.Span{
+		TraceID:       [16]byte{0x1},
+		SpanID:        [8]byte{0x2},
+		TraceState:    "test=a",
+		ParentSpanID:  [8]byte{0x1},
+		Flags:         1,
+		Name:          "span.a",
+		Kind:          telemetry.SpanKindClient,
+		StartTime:     y2k,
+		EndTime:       y2k.Add(time.Second),
+		Attrs:         attrsA,
+		DroppedAttrs:  2,
+		Events:        []*telemetry.SpanEvent{event},
+		DroppedEvents: 3,
+		Links:         []*telemetry.SpanLink{link},
+		DroppedLinks:  4,
+		Status: &telemetry.Status{
+			Message: "okay",
+			Code:    telemetry.StatusCodeOK,
+		},
+	}
+	pSpanA = func() ptrace.Span {
+		s := ptrace.NewSpan()
+		s.SetTraceID(pcommon.TraceID([16]byte{0x1}))
+		s.SetSpanID(pcommon.SpanID([8]byte{0x2}))
+
+		ts := s.TraceState()
+		ts.FromRaw("test=a")
+
+		s.SetParentSpanID(pcommon.SpanID([8]byte{0x1}))
+		s.SetFlags(1)
+		s.SetName("span.a")
+		s.SetKind(ptrace.SpanKindClient)
+		s.SetStartTimestamp(pcommon.NewTimestampFromTime(y2k))
+		s.SetEndTimestamp(pcommon.NewTimestampFromTime(y2k.Add(time.Second)))
+		pAttrsA.CopyTo(s.Attributes())
+		s.SetDroppedAttributesCount(2)
+		prevent.CopyTo(s.Events().AppendEmpty())
+		s.SetDroppedEventsCount(3)
+		pLink.CopyTo(s.Links().AppendEmpty())
+		s.SetDroppedLinksCount(4)
+
+		stat := s.Status()
+		stat.SetMessage("okay")
+		stat.SetCode(ptrace.StatusCodeOk)
+
+		return s
+	}()
+	schema100 = "http://go.opentelemetry.io/schema/v1.0.0"
+
+	scope = &telemetry.Scope{
+		Name:         "go.opentelemetry.io/otel/trace/internal/telemetry/test",
+		Version:      "v0.0.1",
+		Attrs:        []telemetry.Attr{telemetry.String("department", "ops")},
+		DroppedAttrs: 1,
+	}
+	pScope = func() pcommon.InstrumentationScope {
+		s := pcommon.NewInstrumentationScope()
+		s.SetName("go.opentelemetry.io/otel/trace/internal/telemetry/test")
+		s.SetVersion("v0.0.1")
+		s.Attributes().PutStr("department", "ops")
+		s.SetDroppedAttributesCount(1)
+		return s
+	}()
+
+	scopeSpans = &telemetry.ScopeSpans{
+		Scope:     scope,
+		Spans:     []*telemetry.Span{spanA},
+		SchemaURL: schema100,
+	}
+	pScopeSpans = func() ptrace.ScopeSpans {
+		s := ptrace.NewScopeSpans()
+		pSpanA.CopyTo(s.Spans().AppendEmpty())
+		pScope.CopyTo(s.Scope())
+		s.SetSchemaUrl(schema100)
+		return s
+	}()
+
+	res = telemetry.Resource{
+		Attrs: []telemetry.Attr{
+			telemetry.String("host", "hal"),
+			telemetry.Int("id", 42),
+		},
+		DroppedAttrs: 100,
+	}
+	press = func() pcommon.Resource {
+		r := pcommon.NewResource()
+		r.Attributes().PutStr("host", "hal")
+		r.Attributes().PutInt("id", 42)
+		r.SetDroppedAttributesCount(100)
+		return r
+	}()
+
+	resSpans = &telemetry.ResourceSpans{
+		Resource:   res,
+		SchemaURL:  schema100,
+		ScopeSpans: []*telemetry.ScopeSpans{scopeSpans},
+	}
+	pResSpans = func() ptrace.ResourceSpans {
+		rs := ptrace.NewResourceSpans()
+		press.CopyTo(rs.Resource())
+		pScopeSpans.CopyTo(rs.ScopeSpans().AppendEmpty())
+		rs.SetSchemaUrl(schema100)
+		return rs
+	}()
+
+	traces = telemetry.Traces{
+		ResourceSpans: []*telemetry.ResourceSpans{
+			resSpans,
+		},
+	}
+	pTraces = func() ptrace.Traces {
+		traces := ptrace.NewTraces()
+		pResSpans.CopyTo(traces.ResourceSpans().AppendEmpty())
+		return traces
+	}()
+)
+
+func TestDecode(t *testing.T) {
+	var enc ptrace.JSONMarshaler
+	b, err := enc.MarshalTraces(pTraces)
+	require.NoError(t, err)
+
+	t.Log(string(b)) // This helps when test fails to understand what is being decoded.
+
+	var got telemetry.Traces
+	dec := json.NewDecoder(bytes.NewReader(b))
+	require.NoError(t, dec.Decode(&got))
+
+	assert.Equal(t, traces, got)
+}
+
+func TestEncode(t *testing.T) {
+	var buf bytes.Buffer
+	enc := json.NewEncoder(&buf)
+	require.NoError(t, enc.Encode(traces))
+
+	data := buf.Bytes()
+	t.Log(string(data)) // This helps when test fails to understand what how the data has been encoded.
+
+	var dec ptrace.JSONUnmarshaler
+	got, err := dec.UnmarshalTraces(data)
+	require.NoError(t, err)
+
+	assert.Equal(t, pTraces, got)
+}

trace/internal/telemetry/test/doc.go

@@ -0,0 +1,7 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package test provides isolated testing for
+// go.opentelemetry.io/ote/trace/internal/telemetry. It exists to ensure that
+// package is tested without importing any OTLP or pdata dependencies.
+package test // import "go.opentelemetry.io/otel/trace/internal/telemetry/test"

trace/internal/telemetry/test/go.mod

@@ -0,0 +1,35 @@
+module go.opentelemetry.io/otel/trace/internal/telemetry/test
+
+go 1.22.0
+
+require (
+	github.com/stretchr/testify v1.10.0
+	go.opentelemetry.io/collector/pdata v1.24.0
+	go.opentelemetry.io/otel/trace v1.32.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/gogo/protobuf v1.3.2 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rogpeppe/go-internal v1.13.1 // indirect
+	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
+	golang.org/x/net v0.34.0 // indirect
+	golang.org/x/sys v0.29.0 // indirect
+	golang.org/x/text v0.21.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20250122153221-138b5a5a4fd4 // indirect
+	google.golang.org/grpc v1.69.4 // indirect
+	google.golang.org/protobuf v1.36.3 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
+
+replace go.opentelemetry.io/otel/trace => ../../..
+
+replace go.opentelemetry.io/otel => ../../../..
+
+replace go.opentelemetry.io/otel/metric => ../../../../metric

trace/internal/telemetry/test/go.sum

@@ -0,0 +1,94 @@
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trace/internal/telemetry/traces.go

@@ -0,0 +1,189 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// Traces represents the traces data that can be stored in a persistent storage,
+// OR can be embedded by other protocols that transfer OTLP traces data but do
+// not implement the OTLP protocol.
+//
+// The main difference between this message and collector protocol is that
+// in this message there will not be any "control" or "metadata" specific to
+// OTLP protocol.
+//
+// When new fields are added into this message, the OTLP request MUST be updated
+// as well.
+type Traces struct {
+	// An array of ResourceSpans.
+	// For data coming from a single resource this array will typically contain
+	// one element. Intermediary nodes that receive data from multiple origins
+	// typically batch the data before forwarding further and in that case this
+	// array will contain multiple elements.
+	ResourceSpans []*ResourceSpans `json:"resourceSpans,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into td.
+func (td *Traces) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid TracesData type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid TracesData field: %#v", keyIface)
+		}
+
+		switch key {
+		case "resourceSpans", "resource_spans":
+			err = decoder.Decode(&td.ResourceSpans)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// A collection of ScopeSpans from a Resource.
+type ResourceSpans struct {
+	// The resource for the spans in this message.
+	// If this field is not set then no resource info is known.
+	Resource Resource `json:"resource"`
+	// A list of ScopeSpans that originate from a resource.
+	ScopeSpans []*ScopeSpans `json:"scopeSpans,omitempty"`
+	// This schema_url applies to the data in the "resource" field. It does not apply
+	// to the data in the "scope_spans" field which have their own schema_url field.
+	SchemaURL string `json:"schemaUrl,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into rs.
+func (rs *ResourceSpans) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid ResourceSpans type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid ResourceSpans field: %#v", keyIface)
+		}
+
+		switch key {
+		case "resource":
+			err = decoder.Decode(&rs.Resource)
+		case "scopeSpans", "scope_spans":
+			err = decoder.Decode(&rs.ScopeSpans)
+		case "schemaUrl", "schema_url":
+			err = decoder.Decode(&rs.SchemaURL)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// A collection of Spans produced by an InstrumentationScope.
+type ScopeSpans struct {
+	// The instrumentation scope information for the spans in this message.
+	// Semantically when InstrumentationScope isn't set, it is equivalent with
+	// an empty instrumentation scope name (unknown).
+	Scope *Scope `json:"scope"`
+	// A list of Spans that originate from an instrumentation scope.
+	Spans []*Span `json:"spans,omitempty"`
+	// The Schema URL, if known. This is the identifier of the Schema that the span data
+	// is recorded in. To learn more about Schema URL see
+	// https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
+	// This schema_url applies to all spans and span events in the "spans" field.
+	SchemaURL string `json:"schemaUrl,omitempty"`
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into ss.
+func (ss *ScopeSpans) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid ScopeSpans type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid ScopeSpans field: %#v", keyIface)
+		}
+
+		switch key {
+		case "scope":
+			err = decoder.Decode(&ss.Scope)
+		case "spans":
+			err = decoder.Decode(&ss.Spans)
+		case "schemaUrl", "schema_url":
+			err = decoder.Decode(&ss.SchemaURL)
+		default:
+			// Skip unknown.
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}

trace/internal/telemetry/traces_test.go

@@ -0,0 +1,145 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry
+
+import (
+	"testing"
+	"time"
+)
+
+func TestTracesEncoding(t *testing.T) {
+	traces := &Traces{
+		ResourceSpans: []*ResourceSpans{{}},
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(traces, []byte(`{
+		"resourceSpans": [
+			{
+				"resource": {}
+			}
+		]
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(traces, []byte(`{
+		"resource_spans": [
+			{
+				"resource": {}
+			}
+		]
+	}`)))
+}
+
+func TestResourceSpansEncoding(t *testing.T) {
+	rs := &ResourceSpans{
+		Resource: Resource{
+			Attrs: []Attr{String("key", "val")},
+		},
+		ScopeSpans: []*ScopeSpans{{}},
+		SchemaURL:  schema100,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(rs, []byte(`{
+		"resource": {
+			"attributes": [
+				{
+					"key": "key",
+					"value": {
+						"stringValue": "val"
+					}
+				}
+			]
+		},
+		"scopeSpans": [
+			{
+				"scope": null
+			}
+		],
+		"schemaUrl": "http://go.opentelemetry.io/schema/v1.0.0"
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(rs, []byte(`{
+		"resource": {
+			"attributes": [
+				{
+					"key": "key",
+					"value": {
+						"string_value": "val"
+					}
+				}
+			]
+		},
+		"scope_spans": [
+			{
+				"scope": null
+			}
+		],
+		"schema_url": "http://go.opentelemetry.io/schema/v1.0.0"
+	}`)))
+}
+
+func TestScopeSpansEncoding(t *testing.T) {
+	ss := &ScopeSpans{
+		Scope: &Scope{Name: "scope"},
+		Spans: []*Span{{
+			TraceID:   [16]byte{0x1},
+			SpanID:    [8]byte{0x2},
+			Name:      "A",
+			StartTime: y2k,
+			EndTime:   y2k.Add(time.Second),
+		}, {
+			TraceID:   [16]byte{0x1},
+			SpanID:    [8]byte{0x3},
+			Name:      "B",
+			StartTime: y2k.Add(time.Second),
+			EndTime:   y2k.Add(2 * time.Second),
+		}},
+		SchemaURL: schema100,
+	}
+
+	t.Run("CamelCase", runJSONEncodingTests(ss, []byte(`{
+		"scope": {
+			"name": "scope"
+		},
+		"spans": [
+			{
+				"traceId": "01000000000000000000000000000000",
+				"spanId": "0200000000000000",
+				"name": "A",
+				"startTimeUnixNano": 946684800000000000,
+				"endTimeUnixNano": 946684801000000000
+			},
+			{
+				"traceId": "01000000000000000000000000000000",
+				"spanId": "0300000000000000",
+				"name": "B",
+				"startTimeUnixNano": 946684801000000000,
+				"endTimeUnixNano": 946684802000000000
+			}
+		],
+		"schemaUrl": "http://go.opentelemetry.io/schema/v1.0.0"
+	}`)))
+
+	t.Run("SnakeCase/Unmarshal", runJSONUnmarshalTest(ss, []byte(`{
+		"scope": {
+			"name": "scope"
+		},
+		"spans": [
+			{
+				"trace_id": "01000000000000000000000000000000",
+				"span_id": "0200000000000000",
+				"name": "A",
+				"start_time_unix_nano": 946684800000000000,
+				"end_time_unix_nano": 946684801000000000
+			},
+			{
+				"trace_id": "01000000000000000000000000000000",
+				"span_id": "0300000000000000",
+				"name": "B",
+				"start_time_unix_nano": 946684801000000000,
+				"end_time_unix_nano": 946684802000000000
+			}
+		],
+		"schema_url": "http://go.opentelemetry.io/schema/v1.0.0"
+	}`)))
+}

trace/internal/telemetry/value.go

@@ -0,0 +1,453 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "go.opentelemetry.io/otel/trace/internal/telemetry"
+
+import (
+	"bytes"
+	"cmp"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"math"
+	"slices"
+	"strconv"
+	"unsafe"
+)
+
+// A Value represents a structured value.
+// A zero value is valid and represents an empty value.
+type Value struct {
+	// Ensure forward compatibility by explicitly making this not comparable.
+	noCmp [0]func() //nolint: unused  // This is indeed used.
+
+	// num holds the value for Int64, Float64, and Bool. It holds the length
+	// for String, Bytes, Slice, Map.
+	num uint64
+	// any holds either the KindBool, KindInt64, KindFloat64, stringptr,
+	// bytesptr, sliceptr, or mapptr. If KindBool, KindInt64, or KindFloat64
+	// then the value of Value is in num as described above. Otherwise, it
+	// contains the value wrapped in the appropriate type.
+	any any
+}
+
+type (
+	// sliceptr represents a value in Value.any for KindString Values.
+	stringptr *byte
+	// bytesptr represents a value in Value.any for KindBytes Values.
+	bytesptr *byte
+	// sliceptr represents a value in Value.any for KindSlice Values.
+	sliceptr *Value
+	// mapptr represents a value in Value.any for KindMap Values.
+	mapptr *Attr
+)
+
+// ValueKind is the kind of a [Value].
+type ValueKind int
+
+// ValueKind values.
+const (
+	ValueKindEmpty ValueKind = iota
+	ValueKindBool
+	ValueKindFloat64
+	ValueKindInt64
+	ValueKindString
+	ValueKindBytes
+	ValueKindSlice
+	ValueKindMap
+)
+
+var valueKindStrings = []string{
+	"Empty",
+	"Bool",
+	"Float64",
+	"Int64",
+	"String",
+	"Bytes",
+	"Slice",
+	"Map",
+}
+
+func (k ValueKind) String() string {
+	if k >= 0 && int(k) < len(valueKindStrings) {
+		return valueKindStrings[k]
+	}
+	return "<unknown telemetry.ValueKind>"
+}
+
+// StringValue returns a new [Value] for a string.
+func StringValue(v string) Value {
+	return Value{
+		num: uint64(len(v)),
+		any: stringptr(unsafe.StringData(v)),
+	}
+}
+
+// IntValue returns a [Value] for an int.
+func IntValue(v int) Value { return Int64Value(int64(v)) }
+
+// Int64Value returns a [Value] for an int64.
+func Int64Value(v int64) Value {
+	return Value{
+		num: uint64(v), // nolint: gosec  // Store raw bytes.
+		any: ValueKindInt64,
+	}
+}
+
+// Float64Value returns a [Value] for a float64.
+func Float64Value(v float64) Value {
+	return Value{num: math.Float64bits(v), any: ValueKindFloat64}
+}
+
+// BoolValue returns a [Value] for a bool.
+func BoolValue(v bool) Value { //nolint:revive // Not a control flag.
+	var n uint64
+	if v {
+		n = 1
+	}
+	return Value{num: n, any: ValueKindBool}
+}
+
+// BytesValue returns a [Value] for a byte slice. The passed slice must not be
+// changed after it is passed.
+func BytesValue(v []byte) Value {
+	return Value{
+		num: uint64(len(v)),
+		any: bytesptr(unsafe.SliceData(v)),
+	}
+}
+
+// SliceValue returns a [Value] for a slice of [Value]. The passed slice must
+// not be changed after it is passed.
+func SliceValue(vs ...Value) Value {
+	return Value{
+		num: uint64(len(vs)),
+		any: sliceptr(unsafe.SliceData(vs)),
+	}
+}
+
+// MapValue returns a new [Value] for a slice of key-value pairs. The passed
+// slice must not be changed after it is passed.
+func MapValue(kvs ...Attr) Value {
+	return Value{
+		num: uint64(len(kvs)),
+		any: mapptr(unsafe.SliceData(kvs)),
+	}
+}
+
+// AsString returns the value held by v as a string.
+func (v Value) AsString() string {
+	if sp, ok := v.any.(stringptr); ok {
+		return unsafe.String(sp, v.num)
+	}
+	// TODO: error handle
+	return ""
+}
+
+// asString returns the value held by v as a string. It will panic if the Value
+// is not KindString.
+func (v Value) asString() string {
+	return unsafe.String(v.any.(stringptr), v.num)
+}
+
+// AsInt64 returns the value held by v as an int64.
+func (v Value) AsInt64() int64 {
+	if v.Kind() != ValueKindInt64 {
+		// TODO: error handle
+		return 0
+	}
+	return v.asInt64()
+}
+
+// asInt64 returns the value held by v as an int64. If v is not of KindInt64,
+// this will return garbage.
+func (v Value) asInt64() int64 {
+	// Assumes v.num was a valid int64 (overflow not checked).
+	return int64(v.num) // nolint: gosec
+}
+
+// AsBool returns the value held by v as a bool.
+func (v Value) AsBool() bool {
+	if v.Kind() != ValueKindBool {
+		// TODO: error handle
+		return false
+	}
+	return v.asBool()
+}
+
+// asBool returns the value held by v as a bool. If v is not of KindBool, this
+// will return garbage.
+func (v Value) asBool() bool { return v.num == 1 }
+
+// AsFloat64 returns the value held by v as a float64.
+func (v Value) AsFloat64() float64 {
+	if v.Kind() != ValueKindFloat64 {
+		// TODO: error handle
+		return 0
+	}
+	return v.asFloat64()
+}
+
+// asFloat64 returns the value held by v as a float64. If v is not of
+// KindFloat64, this will return garbage.
+func (v Value) asFloat64() float64 { return math.Float64frombits(v.num) }
+
+// AsBytes returns the value held by v as a []byte.
+func (v Value) AsBytes() []byte {
+	if sp, ok := v.any.(bytesptr); ok {
+		return unsafe.Slice((*byte)(sp), v.num)
+	}
+	// TODO: error handle
+	return nil
+}
+
+// asBytes returns the value held by v as a []byte. It will panic if the Value
+// is not KindBytes.
+func (v Value) asBytes() []byte {
+	return unsafe.Slice((*byte)(v.any.(bytesptr)), v.num)
+}
+
+// AsSlice returns the value held by v as a []Value.
+func (v Value) AsSlice() []Value {
+	if sp, ok := v.any.(sliceptr); ok {
+		return unsafe.Slice((*Value)(sp), v.num)
+	}
+	// TODO: error handle
+	return nil
+}
+
+// asSlice returns the value held by v as a []Value. It will panic if the Value
+// is not KindSlice.
+func (v Value) asSlice() []Value {
+	return unsafe.Slice((*Value)(v.any.(sliceptr)), v.num)
+}
+
+// AsMap returns the value held by v as a []Attr.
+func (v Value) AsMap() []Attr {
+	if sp, ok := v.any.(mapptr); ok {
+		return unsafe.Slice((*Attr)(sp), v.num)
+	}
+	// TODO: error handle
+	return nil
+}
+
+// asMap returns the value held by v as a []Attr. It will panic if the
+// Value is not KindMap.
+func (v Value) asMap() []Attr {
+	return unsafe.Slice((*Attr)(v.any.(mapptr)), v.num)
+}
+
+// Kind returns the Kind of v.
+func (v Value) Kind() ValueKind {
+	switch x := v.any.(type) {
+	case ValueKind:
+		return x
+	case stringptr:
+		return ValueKindString
+	case bytesptr:
+		return ValueKindBytes
+	case sliceptr:
+		return ValueKindSlice
+	case mapptr:
+		return ValueKindMap
+	default:
+		return ValueKindEmpty
+	}
+}
+
+// Empty returns if v does not hold any value.
+func (v Value) Empty() bool { return v.Kind() == ValueKindEmpty }
+
+// Equal returns if v is equal to w.
+func (v Value) Equal(w Value) bool {
+	k1 := v.Kind()
+	k2 := w.Kind()
+	if k1 != k2 {
+		return false
+	}
+	switch k1 {
+	case ValueKindInt64, ValueKindBool:
+		return v.num == w.num
+	case ValueKindString:
+		return v.asString() == w.asString()
+	case ValueKindFloat64:
+		return v.asFloat64() == w.asFloat64()
+	case ValueKindSlice:
+		return slices.EqualFunc(v.asSlice(), w.asSlice(), Value.Equal)
+	case ValueKindMap:
+		sv := sortMap(v.asMap())
+		sw := sortMap(w.asMap())
+		return slices.EqualFunc(sv, sw, Attr.Equal)
+	case ValueKindBytes:
+		return bytes.Equal(v.asBytes(), w.asBytes())
+	case ValueKindEmpty:
+		return true
+	default:
+		// TODO: error handle
+		return false
+	}
+}
+
+func sortMap(m []Attr) []Attr {
+	sm := make([]Attr, len(m))
+	copy(sm, m)
+	slices.SortFunc(sm, func(a, b Attr) int {
+		return cmp.Compare(a.Key, b.Key)
+	})
+
+	return sm
+}
+
+// String returns Value's value as a string, formatted like [fmt.Sprint].
+//
+// The returned string is meant for debugging;
+// the string representation is not stable.
+func (v Value) String() string {
+	switch v.Kind() {
+	case ValueKindString:
+		return v.asString()
+	case ValueKindInt64:
+		// Assumes v.num was a valid int64 (overflow not checked).
+		return strconv.FormatInt(int64(v.num), 10) // nolint: gosec
+	case ValueKindFloat64:
+		return strconv.FormatFloat(v.asFloat64(), 'g', -1, 64)
+	case ValueKindBool:
+		return strconv.FormatBool(v.asBool())
+	case ValueKindBytes:
+		return fmt.Sprint(v.asBytes())
+	case ValueKindMap:
+		return fmt.Sprint(v.asMap())
+	case ValueKindSlice:
+		return fmt.Sprint(v.asSlice())
+	case ValueKindEmpty:
+		return "<nil>"
+	default:
+		// Try to handle this as gracefully as possible.
+		//
+		// Don't panic here. The goal here is to have developers find this
+		// first if a slog.Kind is is not handled. It is
+		// preferable to have user's open issue asking why their attributes
+		// have a "unhandled: " prefix than say that their code is panicking.
+		return fmt.Sprintf("<unhandled telemetry.ValueKind: %s>", v.Kind())
+	}
+}
+
+// MarshalJSON encodes v into OTLP formatted JSON.
+func (v *Value) MarshalJSON() ([]byte, error) {
+	switch v.Kind() {
+	case ValueKindString:
+		return json.Marshal(struct {
+			Value string `json:"stringValue"`
+		}{v.asString()})
+	case ValueKindInt64:
+		return json.Marshal(struct {
+			Value string `json:"intValue"`
+		}{strconv.FormatInt(int64(v.num), 10)}) // nolint: gosec  // From raw bytes.
+	case ValueKindFloat64:
+		return json.Marshal(struct {
+			Value float64 `json:"doubleValue"`
+		}{v.asFloat64()})
+	case ValueKindBool:
+		return json.Marshal(struct {
+			Value bool `json:"boolValue"`
+		}{v.asBool()})
+	case ValueKindBytes:
+		return json.Marshal(struct {
+			Value []byte `json:"bytesValue"`
+		}{v.asBytes()})
+	case ValueKindMap:
+		return json.Marshal(struct {
+			Value struct {
+				Values []Attr `json:"values"`
+			} `json:"kvlistValue"`
+		}{struct {
+			Values []Attr `json:"values"`
+		}{v.asMap()}})
+	case ValueKindSlice:
+		return json.Marshal(struct {
+			Value struct {
+				Values []Value `json:"values"`
+			} `json:"arrayValue"`
+		}{struct {
+			Values []Value `json:"values"`
+		}{v.asSlice()}})
+	case ValueKindEmpty:
+		return nil, nil
+	default:
+		return nil, fmt.Errorf("unknown Value kind: %s", v.Kind().String())
+	}
+}
+
+// UnmarshalJSON decodes the OTLP formatted JSON contained in data into v.
+func (v *Value) UnmarshalJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	t, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	if t != json.Delim('{') {
+		return errors.New("invalid Value type")
+	}
+
+	for decoder.More() {
+		keyIface, err := decoder.Token()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				// Empty.
+				return nil
+			}
+			return err
+		}
+
+		key, ok := keyIface.(string)
+		if !ok {
+			return fmt.Errorf("invalid Value key: %#v", keyIface)
+		}
+
+		switch key {
+		case "stringValue", "string_value":
+			var val string
+			err = decoder.Decode(&val)
+			*v = StringValue(val)
+		case "boolValue", "bool_value":
+			var val bool
+			err = decoder.Decode(&val)
+			*v = BoolValue(val)
+		case "intValue", "int_value":
+			var val protoInt64
+			err = decoder.Decode(&val)
+			*v = Int64Value(val.Int64())
+		case "doubleValue", "double_value":
+			var val float64
+			err = decoder.Decode(&val)
+			*v = Float64Value(val)
+		case "bytesValue", "bytes_value":
+			var val64 string
+			if err := decoder.Decode(&val64); err != nil {
+				return err
+			}
+			var val []byte
+			val, err = base64.StdEncoding.DecodeString(val64)
+			*v = BytesValue(val)
+		case "arrayValue", "array_value":
+			var val struct{ Values []Value }
+			err = decoder.Decode(&val)
+			*v = SliceValue(val.Values...)
+		case "kvlistValue", "kvlist_value":
+			var val struct{ Values []Attr }
+			err = decoder.Decode(&val)
+			*v = MapValue(val.Values...)
+		default:
+			// Skip unknown.
+			continue
+		}
+		// Use first valid. Ignore the rest.
+		return err
+	}
+
+	// Only unknown fields. Return nil without unmarshaling any value.
+	return nil
+}

trace/noop.go

@@ -82,4 +82,22 @@ func (noopSpan) AddLink(Link) {}
 func (noopSpan) SetName(string) {}
 
 // TracerProvider returns a no-op TracerProvider.
-func (noopSpan) TracerProvider() TracerProvider { return noopTracerProvider{} }
+func (s noopSpan) TracerProvider() TracerProvider {
+	return s.tracerProvider(autoInstEnabled)
+}
+
+// autoInstEnabled defines if the auto-instrumentation SDK is enabled.
+//
+// The auto-instrumentation is expected to overwrite this value to true when it
+// attaches to the process.
+var autoInstEnabled = new(bool)
+
+// tracerProvider return a noopTracerProvider if autoEnabled is false,
+// otherwise it will return a TracerProvider from the sdk package used in
+// auto-instrumentation.
+func (noopSpan) tracerProvider(autoEnabled *bool) TracerProvider {
+	if *autoEnabled {
+		return newAutoTracerProvider()
+	}
+	return noopTracerProvider{}
+}

versions.yaml

@@ -40,3 +40,4 @@ module-sets:
       - go.opentelemetry.io/otel/schema
 excluded-modules:
   - go.opentelemetry.io/otel/internal/tools
+  - go.opentelemetry.io/otel/trace/internal/telemetry/test