Propagate unknown trace flag bits

propagation/trace_context.go

@@ -46,15 +46,12 @@ func (tc TraceContext) Inject(ctx context.Context, carrier TextMapCarrier) {
 		carrier.Set(tracestateHeader, ts)
 	}
 
-	// Clear all flags other than the trace-context supported sampling bit.
-	flags := sc.TraceFlags() & trace.FlagsSampled
-
 	var sb strings.Builder
 	sb.Grow(2 + 32 + 16 + 2 + 3)
 	_, _ = sb.WriteString(versionPart)
 	traceID := sc.TraceID()
 	spanID := sc.SpanID()
-	flagByte := [1]byte{byte(flags)}
+	flagByte := [1]byte{byte(sc.TraceFlags())}
 	var buf [32]byte
 	for _, src := range [][]byte{traceID[:], spanID[:], flagByte[:]} {
 		_ = sb.WriteByte(delimiter[0])

propagation/trace_context_test.go

@@ -288,9 +288,9 @@ func TestInjectValidTraceContext(t *testing.T) {
 			}),
 		},
 		{
-			name: "unsupported trace flag bits dropped",
+			name: "keep unknown trace flag bits",
 			header: http.Header{
-				traceparent: []string{"00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"},
+				traceparent: []string{"00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-ff"},
 			},
 			sc: trace.NewSpanContext(trace.SpanContextConfig{
 				TraceID:    traceID,