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

spring-test/src/test/resources/log4j2-test.xml

@@ -18,6 +18,7 @@
 		<Logger name="org.springframework.test.context.cache" level="warn" />
 		<Logger name="org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate" level="info" />
 		<Logger name="org.springframework.test.context.junit4.rules" level="warn" />
+		<Logger name="org.springframework.test.context.observation" level="warn" />
 		<Logger name="org.springframework.test.context.transaction.TransactionalTestExecutionListener" level="warn" />
 		<Logger name="org.springframework.test.context.web" level="warn" />
 		<!-- The following must be kept at DEBUG in order to test SPR-14363. -->