Extensible MicrometerMetricsCaptor and polishing

- change `loadCaptor` to return any existing captor
- make builders and facades protected to allow subclassing
- change `AbstractMeter.getMeter()` to use generics
- add test case demonstrating meter name changes
- docs

* Polishing - PR Comments, and rebase

src/reference/asciidoc/metrics.adoc

@@ -157,6 +157,9 @@ In addition, there are three `Gauge` Meters:
 * `spring.integration.handlers`: The number of `MessageHandlers` in the application.
 * `spring.integration.sources`: The number of `MessageSources` in the application.
 
+It is possible to customize the names and tags of `Meters` created by integration components by providing a subclass of `MicrometerMetricsCaptor`.
+The https://github.com/spring-projects/spring-integration/blob/master/spring-integration-core/src/test/java/org/springframework/integration/support/management/micrometer/MicrometerCustomMetricsTests.java[MicrometerCustomMetricsTests] test case shows a simple example of how to do that.
+You can also further customize the meters by overloading the `build()` methods on builder subclasses.
 
 [[mgmt-channel-features]]
 ==== `MessageChannel` Metric Features

src/reference/asciidoc/whats-new.adoc

@@ -200,3 +200,9 @@ Object name key values are now quoted if they contain any characters other than
 e.g. `org.springframework.integration:type=MessageChannel,` `name="input:foo.myGroup.errors"`.
 This has the side effect that previously "allowed" names, with such characters, will now be quoted.
 e.g. `org.springframework.integration:type=MessageChannel,` `name="input#foo.myGroup.errors"`.
+
+[[x51.-micrometer]]
+=== Micrometer Support Changes
+
+It is now simpler to customize the standard Micrometer meters created by the framework.
+See <<micrometer-integration>> for more information.