Micrometer registries pin the application context

[LordKay-sudo]

Summary

Related to spring-framework#36737.

• Fix MeterRegistryCloser so it tracks registries added to Metrics.globalRegistry at registration time instead of snapshotting MeterRegistry beans in its constructor (which often runs before registries exist).
• On ContextClosedEvent, remove tracked registries from the global registry and close them.
• Add DisableGlobalMeterRegistryContextCustomizerFactory so tests default management.metrics.use-global-registry=false when not explicitly configured, mirroring the existing Reactor global-resources customizer and preventing cached test contexts from being pinned via Micrometer.

Test plan

• ./gradlew :module:spring-boot-micrometer-metrics:test --tests MeterRegistryPostProcessorTests
• ./gradlew :core:spring-boot-test:test --tests DisableGlobalMeterRegistryContextCustomizerFactoryTests

[mhalbritter]

@LordKay-sudo no need to deal with the formatting changes, i already have the polish commit locally.

Note to myself: This changes the behavior of the global registry in tests: before that PR, the global registry is there, after that PR it's switched off by default. This is an observable change, not sure we should do that in 4.0.x or 4.2.x.

[mhalbritter]

I had to revert the changes for the tests - we're targeting 4.0.x with that fix, and disabling the global registry in tests is too much of a change in a patch release.

Could you please submit the "disable global registry in tests" changes as a new PR, so we can merge that into main (Spring Boot 4.2.x)? Thanks!

[LordKay-sudo]

@mhalbritter Thanks. Opened #51142 against main with only the disable-global-registry-in-tests customizer.

[mhalbritter]

Thank you.