GH-3626: RabbitMQ Stream Support (#3895)

Resolves https://github.com/spring-projects/spring-integration/issues/3626

- spec for stream listener container
- move message handler from scst to here; add spec

* Fix test.

* Polishing and Docs.

* Fix anchors in doc.

* Don't Extend `AbstractMessageProducingHandler`

spring-integration-core/src/main/java/org/springframework/integration/config/IntegrationManagementConfigurer.java

@@ -162,7 +162,6 @@ public class IntegrationManagementConfigurer
 		return this.metricsCaptor;
 	}
 
-	@Nullable
 	private void setupObservationRegistry() {
 		if (this.observationRegistry == null && this.observationRegistryProvider != null) {
 			this.observationRegistry = this.observationRegistryProvider.getIfUnique();