fix: record left status in jenkins.queue.count metric and add queue metrics test

[kanagaabishek]

… metrics test

Issue #1174: The 'left' queue item status was counted in the batchCallback but never recorded to the queueItems ObservableLongMeasurement, making the 'left' status invisible in the jenkins.queue.count metric.

This commit:

• Records left items with STATUS='left' in the queueItems gauge
• Adds MonitoringQueueListenerTest verifying all queue metrics are registered and exported after a job completes

Fixes: #1174

Fixes #1174

The left queue item status was counted inside the batchCallback in
MonitoringQueueListener but was never recorded to the queueItems
ObservableLongMeasurement. This meant the left status was silently
dropped from the jenkins.queue.count metric while all other statuses
(blocked, buildable, stuck, waiting) were correctly recorded.

Root cause

In MonitoringQueueListener.postConstruct(), the batch callback tracks
left items via AtomicInteger left and increments it correctly, but
the corresponding queueItems.record(left.get(), Attributes.of(STATUS, "left"))
call was missing. Every other status had its recording call — left did not.

Changes

• MonitoringQueueListener.java: added queueItems.record(left.get(), Attributes.of(STATUS, "left")) alongside the existing status recordings
• MonitoringQueueListenerTest.java: first dedicated test for queue metrics — verifies all six queue metrics (jenkins.queue.count, jenkins.queue.waiting, jenkins.queue.blocked, jenkins.queue.buildable, jenkins.queue.left, jenkins.queue.time_spent_millis) are registered and exported after a job completes

Testing done

Added MonitoringQueueListenerTest — the first automated test for queue metrics.

The test:

1. Schedules and completes a FreeStyleProject job
2. Forces an SDK meter provider flush via forceFlush().join(10, TimeUnit.SECONDS)
3. Asserts all six queue metrics are present in the exported metrics map
4. Asserts jenkins.queue.left counter value is >= 1 after the job completes

To run:
./mvnw test -Dtest="MonitoringQueueListenerTest"

Test passes locally on Java 21 / Maven 3.9.9.

Submitter checklist

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• Ensure that the pull request title represents the desired changelog entry
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• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[kanagaabishek]

I noticed PR #1240 also addresses #1174 by fixing ACL-restricted queue visibility.

My PR addresses a separate gap: the left status is counted via AtomicInteger left but the corresponding queueItems.record(left.get(), ...) call was never written, so jenkins.queue.count{status="left"} is always absent regardless of ACL context.

These two fixes address different failure modes of the same issue — #1240 fixes visibility under restricted auth, this PR fixes completeness of the left dimension.

Happy to rebase on top of #1240 if the maintainers prefer a combined fix.