make trace.TraceContext as default context propagator (#456)

* make trace.TraceContext as default context propagator

* Update api/trace/trace_context_propagator.go

Co-Authored-By: Krzesimir Nowak <[email protected]>

* Update plugin/othttp/handler.go

* Update DefaultPropagator in plugin/{http,grpc}trace

* update DefaultPropagator as method instead of var

Co-authored-by: Krzesimir Nowak <[email protected]>
Co-authored-by: Rahul Patel <[email protected]>

Author: 3 people

api/trace/trace_context_propagator.go

@@ -42,6 +42,11 @@ type TraceContext struct{}
 var _ propagation.TextFormat = TraceContext{}
 var traceCtxRegExp = regexp.MustCompile("^[0-9a-f]{2}-[a-f0-9]{32}-[a-f0-9]{16}-[a-f0-9]{2}-?")
 
+// DefaultPropagator returns the default trace propagator.
+func DefaultPropagator() propagation.TextFormat {
+	return TraceContext{}
+}
+
 func (hp TraceContext) Inject(ctx context.Context, supplier propagation.Supplier) {
 	sc := SpanFromContext(ctx).SpanContext()
 	if sc.IsValid() {

plugin/grpctrace/grpctrace.go

@@ -24,7 +24,7 @@ import (
 )
 
 var (
-	propagator = trace.TraceContext{}
+	propagator = trace.DefaultPropagator()
 )
 
 type metadataSupplier struct {

plugin/httptrace/httptrace.go

@@ -27,7 +27,7 @@ var (
 	HostKey = key.New("http.host")
 	URLKey  = key.New("http.url")
 
-	propagator = trace.TraceContext{}
+	propagator = trace.DefaultPropagator()
 )
 
 // Returns the Attributes, Context Entries, and SpanContext that were encoded by Inject.

plugin/othttp/handler.go

@@ -79,7 +79,7 @@ func WithPublicEndpoint() Option {
 
 // WithPropagator configures the Handler with a specific propagator. If this
 // option isn't specificed then
-// go.opentelemetry.io/otel/api/trace.TraceContext is used.
+// go.opentelemetry.io/otel/api/trace.DefaultPropagator is used.
 func WithPropagator(p propagation.TextFormat) Option {
 	return func(h *Handler) {
 		h.prop = p
@@ -130,7 +130,7 @@ func NewHandler(handler http.Handler, operation string, opts ...Option) http.Han
 	h := Handler{handler: handler, operation: operation}
 	defaultOpts := []Option{
 		WithTracer(global.TraceProvider().Tracer("go.opentelemetry.io/plugin/othttp")),
-		WithPropagator(trace.TraceContext{}),
+		WithPropagator(trace.DefaultPropagator()),
 		WithSpanOptions(trace.WithSpanKind(trace.SpanKindServer)),
 	}