feat: add tracing metadata in the gRPC client interceptor

This commit introduces an enhancement to the gRPC client interceptor to add tracing metadata. This is achieved by injecting trace context and attaching it to the outgoing metadata of the client requests.

grpc/client_interceptor.go

@@ -24,7 +24,7 @@ func AddDaprAppID(appID string) grpc.UnaryClientInterceptor {
 func AddTraceContext() grpc.UnaryClientInterceptor {
 	return func(ctx context.Context, method string, req, reply interface{},
 		cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
-		var carrier http.Header
+		carrier := make(http.Header)
 		trace.InjectTraceContext(ctx, carrier)
 		ctx = metadata.AppendToOutgoingContext(ctx,
 			trace.TraceparentHeader, carrier.Get(trace.TraceparentHeader))