GH-2729: Reactor Kafka Binder SenderResult Support (#2730)

* GH-2729: Reactor Kafka Binder SenderResult Support

Resolves https://github.com/spring-cloud/spring-cloud-stream/issues/2729

Allow configuration of a `FluxMessageChannel` to receive `SenderResult`s.
Add `SenderResultMessageHandler` to consume from that channel.

Remove undocumented `sendResult` header, which has no value without the
sender result correlation metadata.

* Add integration test and polish property docs.

- result channel must be FMC for reactive binder.

* Add documentation.

* Remove SenderResultMessageHandler.

docs/src/main/asciidoc/kafka/kafka-reactive.adoc

@@ -182,3 +182,48 @@ When `false` (default), a separate binding is created for each topic specified i
 
 Starting with version 4.0.3, the `destination-is-pattern` Kafka binding consumer property is now supported.
 The receiver options are conigured with a regex `Pattern`, allowing the binding to consume from any topic that matches the pattern.
+
+=== Sender Result Channel
+
+Starting with version 4.0.3, you can configure the `resultMetadataChannel` to receive `SenderResult<?>` s to determine success/failure of sends.
+
+The `SenderResult` contains `correlationMetadata` to allow you to correlate results with sends; it also contains `RecordMetadata`, which indicates the `TopicPartition` and offset of the sent record.
+
+The `resultMetadataChannel` **must** be a `FluxMessageChannel` instance.
+
+Here is an example of how to use this feature, with correlation metadata of type `Integer`:
+
+====
+[source, java]
+----
+@Bean
+FluxMessageChannel sendResults() {
+    return new FluxMessageChannel();
+}
+
+@ServiceActivator(inputChannel = "sendResults")
+void handleResults(SenderResult<Integer> result) {
+    if (result.exception() != null) {
+        failureFor(result);
+    }
+    else {
+        successFor(result);
+    }
+}
+----
+====
+
+To set the correlation metadata on an output record, set the `CORRELATION_ID` header:
+
+====
+[source, java]
+----
+streamBridge.send("words1", MessageBuilder.withPayload("foobar")
+        .setCorrelationId(42)
+        .build());
+----
+====
+
+When using the feature with a `Function`, the function output type must be a `Message<?>` with the correlation id header set to the desired value.
+
+Metadata should be unique, at least for the duration of the send.