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
