Add embedded package to trace API (#4620)

* Add trace/embedded

* Update trace impl to use trace/embedded

* Add noop pkg to replace no-op impl in trace pkg

* Use trace/embedded in global impl

* Use trace/embedded in SDK impl

* Update opencensus bridge

* Update opentracing bridge

* Add changes to changelog

* Update trace/doc.go

Co-authored-by: David Ashpole <dashpole@google.com>

---------

Co-authored-by: David Ashpole <dashpole@google.com>

sdk/trace/span.go

@@ -32,6 +32,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/resource"
 	semconv "go.opentelemetry.io/otel/semconv/v1.21.0"
 	"go.opentelemetry.io/otel/trace"
+	"go.opentelemetry.io/otel/trace/embedded"
 )
 
 // ReadOnlySpan allows reading information from the data structure underlying a
@@ -108,6 +109,8 @@ type ReadWriteSpan interface {
 // recordingSpan is an implementation of the OpenTelemetry Span API
 // representing the individual component of a trace that is sampled.
 type recordingSpan struct {
+	embedded.Span
+
 	// mu protects the contents of this span.
 	mu sync.Mutex
 
@@ -774,6 +777,8 @@ func (s *recordingSpan) runtimeTrace(ctx context.Context) context.Context {
 // that wraps a SpanContext. It performs no operations other than to return
 // the wrapped SpanContext or TracerProvider that created it.
 type nonRecordingSpan struct {
+	embedded.Span
+
 	// tracer is the SDK tracer that created this span.
 	tracer *tracer
 	sc     trace.SpanContext