Update rest-grpc-multiplex example to include reflection (#1902)

examples/rest-grpc-multiplex/src/main.rs

@@ -18,6 +18,9 @@ mod multiplex_service;
 
 mod proto {
     tonic::include_proto!("helloworld");
+
+    pub(crate) const FILE_DESCRIPTOR_SET: &[u8] =
+        tonic::include_file_descriptor_set!("helloworld_descriptor");
 }
 
 #[derive(Default)]
@@ -58,7 +61,14 @@ async fn main() {
     let rest = Router::new().route("/", get(web_root));
 
     // build the grpc service
-    let grpc = GreeterServer::new(GrpcServiceImpl::default());
+    let reflection_service = tonic_reflection::server::Builder::configure()
+        .register_encoded_file_descriptor_set(proto::FILE_DESCRIPTOR_SET)
+        .build()
+        .unwrap();
+    let grpc = tonic::transport::Server::builder()
+        .add_service(reflection_service)
+        .add_service(GreeterServer::new(GrpcServiceImpl::default()))
+        .into_service();
 
     // combine them into one service
     let service = MultiplexService::new(rest, grpc);