Fix duplicate metric registration during concurrent pipeline state builds

[patrickmann]

Closes #26080

PipelineInterpreter.State registers cache metrics via a non-atomic removeMatching + safelyRegisterAll sequence. Two independent code paths (PipelineInterpreterStateUpdater and PipelineMetadataUpdateJob) can construct State concurrently, causing Thread B to register metrics between Thread A's remove and register. This results in noisy ERROR logs: Duplicate metric set registered.

The fix adds a static lock to synchronize the compound remove+register operation. This is acceptable since updates are relatively infrequent.

How Tested

Added concurrent regression test StateTest.concurrentStateConstructionDoesNotCauseDuplicateMetricErrors

Manual test: deploy to a test environment, trigger rapid pipeline config changes (e.g. bulk rule updates), verify no "Duplicate metric set registered" ERROR in server logs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[Copilot]

Pull request overview

This PR addresses noisy Duplicate metric set registered ERROR logs caused by a race during concurrent construction of PipelineInterpreter.State, where cache metrics were removed and re-registered via a non-atomic sequence.

Changes:

• Add a static lock in PipelineInterpreter.State to synchronize the metric removeMatching + safelyRegisterAll compound operation.
• Add a concurrent regression test intended to exercise concurrent State construction.
• Add an unreleased changelog entry for issue #26080.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
graylog2-server/src/main/java/org/graylog/plugins/pipelineprocessor/processors/PipelineInterpreter.java	Serializes cache metric remove+register to prevent concurrent duplicate registrations/log spam.
graylog2-server/src/test/java/org/graylog/plugins/pipelineprocessor/processors/StateTest.java	Adds a concurrent construction regression test and switches to MetricRegistry.
changelog/unreleased/issue-26080.toml	Records the fix in the unreleased changelog.

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