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/scope/scope.go

@@ -0,0 +1,67 @@
+package scope
+
+import (
+	"context"
+
+	"github.com/open-telemetry/opentelemetry-go/api/core"
+	"github.com/open-telemetry/opentelemetry-go/exporter/observer"
+)
+
+type (
+	Scope interface {
+		ScopeID() core.ScopeID
+	}
+
+	Mutable interface {
+		Scope
+
+		SetAttribute(core.KeyValue)
+		SetAttributes(...core.KeyValue)
+
+		ModifyAttribute(core.Mutator)
+		ModifyAttributes(...core.Mutator)
+	}
+
+	scopeIdent struct {
+		id core.ScopeID
+	}
+
+	scopeKeyType struct{}
+)
+
+var (
+	scopeKey   = &scopeKeyType{}
+	emptyScope = &scopeIdent{}
+)
+
+func SetActive(ctx context.Context, scope Scope) context.Context {
+	return context.WithValue(ctx, scopeKey, scope)
+}
+
+func Active(ctx context.Context) Scope {
+	if scope, has := ctx.Value(scopeKey).(Scope); has {
+		return scope
+	}
+	return emptyScope
+}
+
+func (s *scopeIdent) ScopeID() core.ScopeID {
+	if s == nil {
+		return core.ScopeID{}
+	}
+	return s.id
+}
+
+func New(parent core.ScopeID, attributes ...core.KeyValue) Scope {
+	eventID := observer.Record(observer.Event{
+		Type:       observer.NEW_SCOPE,
+		Scope:      parent,
+		Attributes: attributes,
+	})
+	return &scopeIdent{
+		id: core.ScopeID{
+			EventID:     eventID,
+			SpanContext: parent.SpanContext,
+		},
+	}
+}