Overhaul TestExecutionListener for Micrometer ObservationRegistry

This commit overhauls the TestExecutionListener for Micrometer's
ObservationRegistry that was introduced in the previous commit.

Specifically, this commit:

- Renames the listener to MicrometerObservationRegistryTestExecutionListener
  since the use of a ThreadLocal is an implementation detail that may
  change over time.

- Makes the listener package-private instead of public in order to
  allow the team greater flexibility in evolving this feature.

- Eagerly loads the ObservationThreadLocalAccessor class and verifies
  that it has a getObservationRegistry() method to ensure that the
  listener is properly skipped when SpringFactoriesLoader attempts to
  load it, if Micrometer 1.10.8+ is not on the classpath.

- Switches the listener's automatic registration order to 2500 in order
  to register it after the DependencyInjectionTestExecutionListener.

- Only tracks the previous ObservationRegistry in beforeTestMethod() if
  the test's ApplicationContext contains an ObservationRegistry bean.

- Properly removes the TestContext attribute for the previous
  ObservationRegistry in afterTestMethod().

- Introduces DEBUG logging for diagnostics.

- Adds an entry in the Javadoc for TestExecutionListener as well as in
  the Testing chapter in the reference manual.

Closes gh-30658

framework-docs/modules/ROOT/pages/testing/testcontext-framework/tel-config.adoc

@@ -12,6 +12,8 @@ by default, exactly in the following order:
   xref:testing/testcontext-framework/application-events.adoc[`ApplicationEvents`].
 * `DependencyInjectionTestExecutionListener`: Provides dependency injection for the test
   instance.
+* `MicrometerObservationRegistryTestExecutionListener`: Provides support for
+  Micrometer's `ObservationRegistry`.
 * `DirtiesContextTestExecutionListener`: Handles the `@DirtiesContext` annotation for
   "`after`" modes.
 * `TransactionalTestExecutionListener`: Provides transactional test execution with