Add metadataPush support to RSocketRequester

Closes gh-24322

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketRequester.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -248,6 +248,11 @@ final class DefaultRSocketRequester implements RSocketRequester {
 					.doOnDiscard(Payload.class, Payload::release);
 		}
 
+		@Override
+		public Mono<Void> sendMetadata() {
+			return getPayloadMono().flatMap(rsocket::metadataPush);
+		}
+
 		@Override
 		public Mono<Void> send() {
 			return getPayloadMono().flatMap(rsocket::fireAndForget);

spring-messaging/src/main/java/org/springframework/messaging/rsocket/RSocketRequester.java

@@ -276,6 +276,12 @@ public interface RSocketRequester {
 		 */
 		RequestSpec metadata(Consumer<MetadataSpec<?>> configurer);
 
+		/**
+		 * Perform a {@link RSocket#metadataPush(Payload) metadataPush}.
+		 * @since 5.3
+		 */
+		Mono<Void> sendMetadata();
+
 		/**
 		 * Provide payload data for the request. This can be one of:
 		 * <ul>
@@ -344,7 +350,12 @@ public interface RSocketRequester {
 	interface RetrieveSpec {
 
 		/**
-		 * Perform a {@link RSocket#fireAndForget fireAndForget}.
+		 * Perform a {@link RSocket#fireAndForget fireAndForget} sending the
+		 * provided data and metadata.
+		 * @return a completion that indicates if the payload was sent
+		 * successfully or not. Note, however that is a one-way send and there
+		 * is no indication of whether or how the even was handled on the
+		 * remote end.
 		 */
 		Mono<Void> send();