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/trace/api.go

@@ -0,0 +1,144 @@
+package trace
+
+import (
+	"context"
+	"time"
+
+	"github.com/open-telemetry/opentelemetry-go/api/core"
+	"github.com/open-telemetry/opentelemetry-go/api/log"
+	"github.com/open-telemetry/opentelemetry-go/api/scope"
+	"github.com/open-telemetry/opentelemetry-go/api/stats"
+	"github.com/open-telemetry/opentelemetry-go/api/tag"
+)
+
+type (
+	Tracer interface {
+		Start(context.Context, string, ...Option) (context.Context, Span)
+
+		WithSpan(
+			ctx context.Context,
+			operation string,
+			body func(ctx context.Context) error,
+		) error
+
+		WithService(name string) Tracer
+		WithComponent(name string) Tracer
+		WithResources(res ...core.KeyValue) Tracer
+
+		// Note: see https://github.com/opentracing/opentracing-go/issues/127
+		Inject(context.Context, Span, Injector)
+
+		// ScopeID returns the resource scope of this tracer.
+		scope.Scope
+	}
+
+	Span interface {
+		scope.Mutable
+
+		log.Interface
+
+		stats.Interface
+
+		SetError(bool)
+
+		Tracer() Tracer
+
+		Finish()
+	}
+
+	Injector interface {
+		Inject(core.SpanContext, tag.Map)
+	}
+
+	Option struct {
+		attribute  core.KeyValue
+		attributes []core.KeyValue
+		startTime  time.Time
+		reference  Reference
+	}
+
+	Reference struct {
+		core.SpanContext
+		RelationshipType
+	}
+
+	RelationshipType int
+)
+
+const (
+	ChildOfRelationship RelationshipType = iota
+	FollowsFromRelationship
+)
+
+func GlobalTracer() Tracer {
+	if t := global.Load(); t != nil {
+		return t.(Tracer)
+	}
+	return empty
+}
+
+func SetGlobalTracer(t Tracer) {
+	global.Store(t)
+}
+
+func Start(ctx context.Context, name string, opts ...Option) (context.Context, Span) {
+	return GlobalTracer().Start(ctx, name, opts...)
+}
+
+func Active(ctx context.Context) Span {
+	span, _ := scope.Active(ctx).(*span)
+	return span
+}
+
+func WithSpan(ctx context.Context, name string, body func(context.Context) error) error {
+	return GlobalTracer().WithSpan(ctx, name, body)
+}
+
+func SetError(ctx context.Context, v bool) {
+	Active(ctx).SetError(v)
+}
+
+func Inject(ctx context.Context, injector Injector) {
+	span := Active(ctx)
+	if span == nil {
+		return
+	}
+
+	span.Tracer().Inject(ctx, span, injector)
+}
+
+func WithStartTime(t time.Time) Option {
+	return Option{
+		startTime: t,
+	}
+}
+
+func WithAttributes(attrs ...core.KeyValue) Option {
+	return Option{
+		attributes: attrs,
+	}
+}
+
+func WithAttribute(attr core.KeyValue) Option {
+	return Option{
+		attribute: attr,
+	}
+}
+
+func ChildOf(sc core.SpanContext) Option {
+	return Option{
+		reference: Reference{
+			SpanContext:      sc,
+			RelationshipType: ChildOfRelationship,
+		},
+	}
+}
+
+func FollowsFrom(sc core.SpanContext) Option {
+	return Option{
+		reference: Reference{
+			SpanContext:      sc,
+			RelationshipType: FollowsFromRelationship,
+		},
+	}
+}