enhance log for hello world SayHello (#2538)

This PR adds a log to `SayHello`. This makes server logic more clear.

examples/helloworld/greeter_server/main.go

@@ -39,6 +39,7 @@ type server struct{}
 
 // SayHello implements helloworld.GreeterServer
 func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
+	log.Printf("Received: %v", in.Name)
 	return &pb.HelloReply{Message: "Hello " + in.Name}, nil
 }