Add observation to inbound endpoints (#3930)

* Add observation to inbound endpoints

* * Improve `Observation` logic in the `MessageProducerSupport` and `MessagingGatewaySupport`
* Clean up Javadocs in observation support classes
* Add test for `MessagingGatewaySupport` instrumentation into `IntegrationObservabilityZipkinTests`
* Add docs for `@EnableIntegrationManagement.observationPatterns()`
* The generated docs for metrics and spans looks OK: have both `Gateway` and `Handler` section now

* * Fix Checkstyle violations

* * Fix `IntegrationMBeanExporter` to filter out a `MessageProducer` from sources.
Marking a `MessageProducerSupport` with an `IntegrationInboundManagement`
causes it to be considered as a source for JMX.
Technically it might have a reason since it is indeed a source, but that's fully different story

* * No observation instrumentation by default

* * Fix "no observation by default" configuration logic

spring-integration-jmx/src/main/java/org/springframework/integration/monitor/IntegrationMBeanExporter.java

@@ -26,6 +26,7 @@ import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Predicate;
 import java.util.stream.Collectors;
 
 import javax.lang.model.SourceVersion;
@@ -268,6 +269,7 @@ public class IntegrationMBeanExporter extends MBeanExporter
 				.stream()
 				// If the source is proxied, we have to extract the target to expose as an MBean.
 				// The MetadataMBeanInfoAssembler does not support JDK dynamic proxies.
+				.filter(Predicate.not(MessageProducer.class::isInstance))
 				.map(this::extractTarget)
 				.map(IntegrationInboundManagement.class::cast)
 				.forEach(src -> this.sources.put(src.getComponentName(), src));