metadata: split context to incoming and outgoing

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

tracer/context.go

@@ -14,11 +14,12 @@ const (
 
 // FromContext returns a span from context
 func FromContext(ctx context.Context) (traceID string, parentSpanID string, isFound bool) {
-	if ctx == nil {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
 		return "", "", false
 	}
-	traceID, traceOk := metadata.Get(ctx, traceIDKey)
-	microID, microOk := metadata.Get(ctx, "Micro-Id")
+	traceID, traceOk := md.Get(traceIDKey)
+	microID, microOk := md.Get("Micro-Id")
 	if !traceOk && !microOk {
 		isFound = false
 		return
@@ -26,17 +27,17 @@ func FromContext(ctx context.Context) (traceID string, parentSpanID string, isFo
 	if !traceOk {
 		traceID = microID
 	}
-	parentSpanID, ok := metadata.Get(ctx, spanIDKey)
+	parentSpanID, ok = md.Get(spanIDKey)
 	return traceID, parentSpanID, ok
 }
 
 // NewContext saves the trace and span ids in the context
 func NewContext(ctx context.Context, traceID, parentSpanID string) context.Context {
-	if ctx == nil {
-		ctx = context.Background()
+	md, ok := metadata.FromContext(ctx)
+	if !ok {
+		md = metadata.New(2)
 	}
-	md := metadata.New(2)
 	md.Set(traceIDKey, traceID)
 	md.Set(spanIDKey, parentSpanID)
-	return metadata.MergeContext(ctx, md, true)
+	return metadata.NewContext(ctx, md)
 }