GH-2956: Add MessageHandler.getMetricsCaptor()

Fixes #2956

* Add `getMetricsCaptor.getMetricsCaptor()` for possible customization in the target implementation
* Added dependancy for `@Nullable`

**Cherry-pick to 5.1.x**

(cherry picked from commit 45fe5be)

Author: amitsadafule

spring-integration-core/src/main/java/org/springframework/integration/handler/AbstractMessageHandler.java

@@ -37,6 +37,7 @@
 import org.springframework.integration.support.management.metrics.SampleFacade;
 import org.springframework.integration.support.management.metrics.TimerFacade;
 import org.springframework.integration.support.utils.IntegrationUtils;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.util.Assert;
@@ -53,6 +54,7 @@
  * @author Oleg Zhurakousky
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Amit Sadafule
  */
 @IntegrationManagedResource
 public abstract class AbstractMessageHandler extends IntegrationObjectSupport
@@ -99,6 +101,11 @@ public void registerMetricsCaptor(MetricsCaptor metricsCaptor) {
 		this.metricsCaptor = metricsCaptor;
 	}
 
+	@Nullable
+	protected MetricsCaptor getMetricsCaptor() {
+		return this.metricsCaptor;
+	}
+
 	@Override
 	public void setOrder(int order) {
 		this.order = order;