Golang opentelemetry-go draft API (incomplete) (#9)

* Work in progress from https://github.com/lightstep/opentelemetry-golang-prototype

* Renames

* Rename

* Finish rename

* Rename packages

* README

api/core/core.go

@@ -0,0 +1,179 @@
+package core
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/open-telemetry/opentelemetry-go/api/unit"
+)
+
+type (
+	ScopeID struct {
+		EventID
+		SpanContext
+	}
+
+	SpanContext struct {
+		TraceIDHigh uint64
+		TraceIDLow  uint64
+		SpanID      uint64
+	}
+
+	EventID uint64
+
+	BaseMeasure interface {
+		Name() string
+		Description() string
+		Unit() unit.Unit
+
+		DefinitionID() EventID
+	}
+
+	Measure interface {
+		BaseMeasure
+
+		M(float64) Measurement
+		V(float64) KeyValue
+	}
+
+	Measurement struct {
+		// NOTE: If we add a ScopeID field this can carry
+		// pre-aggregated measures via the stats.Record API.
+		Measure Measure
+		Value   float64
+		ScopeID ScopeID
+	}
+
+	Key interface {
+		BaseMeasure
+
+		Value(ctx context.Context) KeyValue
+
+		Bool(v bool) KeyValue
+
+		Int(v int) KeyValue
+		Int32(v int32) KeyValue
+		Int64(v int64) KeyValue
+
+		Uint(v uint) KeyValue
+		Uint32(v uint32) KeyValue
+		Uint64(v uint64) KeyValue
+
+		Float32(v float32) KeyValue
+		Float64(v float64) KeyValue
+
+		String(v string) KeyValue
+		Bytes(v []byte) KeyValue
+	}
+
+	KeyValue struct {
+		Key   Key
+		Value Value
+	}
+
+	ValueType int
+
+	Value struct {
+		Type    ValueType
+		Bool    bool
+		Int64   int64
+		Uint64  uint64
+		Float64 float64
+		String  string
+		Bytes   []byte
+
+		// TODO Lazy value type?
+	}
+
+	MutatorOp int
+
+	Mutator struct {
+		MutatorOp
+		KeyValue
+		MeasureMetadata
+	}
+
+	MeasureMetadata struct {
+		MaxHops int // -1 == infinite, 0 == do not propagate
+
+		// TODO time to live?
+	}
+)
+
+const (
+	INVALID ValueType = iota
+	BOOL
+	INT32
+	INT64
+	UINT32
+	UINT64
+	FLOAT32
+	FLOAT64
+	STRING
+	BYTES
+
+	INSERT MutatorOp = iota
+	UPDATE
+	UPSERT
+	DELETE
+)
+
+func (sc SpanContext) HasTraceID() bool {
+	return sc.TraceIDHigh != 0 || sc.TraceIDLow != 0
+}
+
+func (sc SpanContext) HasSpanID() bool {
+	return sc.SpanID != 0
+}
+
+func (sc SpanContext) SpanIDString() string {
+	p := fmt.Sprintf("%.16x", sc.SpanID)
+	return p[0:3] + ".." + p[13:16]
+}
+
+func (sc SpanContext) TraceIDString() string {
+	p1 := fmt.Sprintf("%.16x", sc.TraceIDHigh)
+	p2 := fmt.Sprintf("%.16x", sc.TraceIDLow)
+	return p1[0:3] + ".." + p2[13:16]
+}
+
+// TODO make this a lazy one-time conversion.
+func (v Value) Emit() string {
+	switch v.Type {
+	case BOOL:
+		return fmt.Sprint(v.Bool)
+	case INT32, INT64:
+		return fmt.Sprint(v.Int64)
+	case UINT32, UINT64:
+		return fmt.Sprint(v.Uint64)
+	case FLOAT32, FLOAT64:
+		return fmt.Sprint(v.Float64)
+	case STRING:
+		return v.String
+	case BYTES:
+		return string(v.Bytes)
+	}
+	return "unknown"
+}
+
+func (m Mutator) WithMaxHops(hops int) Mutator {
+	m.MaxHops = hops
+	return m
+}
+
+func (e EventID) Scope() ScopeID {
+	return ScopeID{
+		EventID: e,
+	}
+}
+
+func (s SpanContext) Scope() ScopeID {
+	return ScopeID{
+		SpanContext: s,
+	}
+}
+
+func (m Measurement) With(id ScopeID) Measurement {
+	m.ScopeID = id
+	return m
+}