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**

Author: amitsadafule

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

@@ -35,6 +35,7 @@
 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;
@@ -51,6 +52,7 @@
  * @author Oleg Zhurakousky
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Amit Sadafule
  */
 @SuppressWarnings("deprecation")
 @IntegrationManagedResource
@@ -100,6 +102,11 @@ public void registerMetricsCaptor(MetricsCaptor metricsCaptorToRegister) {
 		this.metricsCaptor = metricsCaptorToRegister;
 	}
 
+	@Nullable
+	protected MetricsCaptor getMetricsCaptor() {
+		return this.metricsCaptor;
+	}
+
 	@Override
 	public void setOrder(int order) {
 		this.order = order;