What is the purpose of the change?

Fixes #16149.

A metadata retry holds the instance listener monitor while fetching metadata from providers. If that request stalls, registry callbacks cannot update the provider list and subsequent retries continue using obsolete instances.

Update instance lists under a short synchronized section and let one refresh processor retrieve metadata from a snapshot. Notifications received during retrieval update the latest snapshot and return; obsolete results are discarded before address publication. Pending changes are also processed after a retrieval failure. Destroyed listeners reject late results and release queued instance events, including when destruction races with the next refresh iteration.

Validation on Linux with JDK 17 and Maven 3.9.9:

• Reproduced the issue before the fix: a provider-change callback timed out while a retry metadata fetch was paused.
• Passed 39 tests covering concurrent provider changes, obsolete snapshots with multiple revisions, repeated notification failures, event acceptance, destruction, existing listeners with both empty-protection settings, and service discovery registry behavior.
• Passed module spotless:check and git diff --check.

The test command was:

mvn -B -ntp -pl dubbo-registry/dubbo-registry-api -am test jacoco:report -Dtest=ServiceInstancesChangedListenerConcurrencyTest,ServiceInstancesChangedListenerTest,ServiceInstancesChangedListenerWithoutEmptyProtectTest,ServiceDiscoveryRegistryTest -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false -Pjacoco,skip-spotless -Dcheckstyle.skip=true -Drat.skip=true

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction.
• Make sure GitHub Actions can pass.