Remove Legacy Metrics

- Simplify MBeans - instead of wrapping to expose lifecycle methods,
  implement `ManageableLifecycle`. Register an additional MBean for
  polled endpoints to control the lifecycle.

* Polishing

- Move `QueueChannel` `@ManagedAttribute`s to `QueueChannelOperations`
- Make all `AbstractEndpoints` `IntegrationManagedResource`s and remove `ManagedEndpoint`
  to allow exposure of any `@Managed*` methods (including those on `Pausable`)
- Revert to `Lifecycle` for classes that are not related to endpoints
- Remove legacy metrics from docs

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/dsl/FtpTests.java

@@ -144,11 +144,7 @@ public class FtpTests extends FtpTestSupport {
 		MessageSource<?> source = context.getBean(FtpInboundFileSynchronizingMessageSource.class);
 		assertThat(TestUtils.getPropertyValue(source, "maxFetchSize")).isEqualTo(10);
 
-		assertThat(this.integrationManagementConfigurer.getSourceMetrics("ftpInboundAdapter.source")).isNotNull();
-
 		registration.destroy();
-
-		assertThat(this.integrationManagementConfigurer.getSourceMetrics("ftpInboundAdapter.source")).isNull();
 	}
 
 	@Test