INT-3831: Fix MessagingGatewaySupport MBean Export

JIRA: https://jira.spring.io/browse/INT-3831

- Add `@IntegrationManagedResource`
- Suppress exporting as an endpoint

Fix `outputChannel` issue for anonymous `MessagingGatewaySupport`

INT-3831: Pure JavaConfig Test

INT-3831: Polishing

Check output channel name for anonymous message source metrics.

Don't use getter on MGS to avoid early channel resolution.

INT-3831: Fix Test - Avoid Second MBeanServer

Reference the `MBSFB` in the context rather than create a new `MBeanServer`.

Polishing

spring-integration-core/src/main/java/org/springframework/integration/channel/NullChannel.java

@@ -225,4 +225,9 @@ public class NullChannel implements PollableChannel, MessageChannelMetrics,
 		return this.receive();
 	}
 
+	@Override
+	public String toString() {
+		return (this.beanName != null) ? this.beanName : super.toString();
+	}
+
 }

spring-integration-core/src/main/java/org/springframework/integration/gateway/MessagingGatewaySupport.java

@@ -30,6 +30,7 @@ import org.springframework.integration.mapping.OutboundMessageMapper;
 import org.springframework.integration.support.DefaultMessageBuilderFactory;
 import org.springframework.integration.support.MessageBuilderFactory;
 import org.springframework.integration.support.converter.SimpleMessageConverter;
+import org.springframework.integration.support.management.IntegrationManagedResource;
 import org.springframework.integration.support.management.MessageSourceMetrics;
 import org.springframework.integration.support.management.TrackableComponent;
 import org.springframework.messaging.Message;
@@ -50,6 +51,7 @@ import org.springframework.util.Assert;
  * @author Gary Russell
  * @author Artem Bilan
  */
+@IntegrationManagedResource
 public abstract class MessagingGatewaySupport extends AbstractEndpoint
 		implements TrackableComponent, MessageSourceMetrics {
 
@@ -313,7 +315,7 @@ public abstract class MessagingGatewaySupport extends AbstractEndpoint
 		}
 	}
 
-	protected MessageChannel getRequestChannel() {
+	public MessageChannel getRequestChannel() {
 		if (this.requestChannelName != null) {
 			synchronized (this) {
 				if (this.requestChannelName != null) {