feat(executor): add garbage collector Prometheus metrics

[davidlin20dev]

Tracking issue

Related to #7455 (part of #7445; does not close the issue).

Why are the changes needed?

The garbage collector emits no metrics, so there is no visibility into whether terminal-TaskAction cleanup is happening, failing, or keeping up. A broken or slow GC silently lets TaskAction CRDs pile up in etcd until the cluster is already degraded.

What changes were proposed in this pull request?

Three Prometheus instruments under a dedicated executor:gc: sub-scope, created once in the constructor and updated each sweep:

• objects_deleted (counter): per successful delete.
• deletion_errors (counter): per failed delete.
• sweep_duration (stopwatch): full-sweep duration, timed via defer.

The scope is injected through NewGarbageCollector(..., scope) from setup.go (executorScope.NewSubScope("gc")). No labels, to keep cardinality bounded.

How was this patch tested?

New unit tests in garbage_collector_metrics_test.go (controller-runtime fake client, no envtest):

• TestGarbageCollectorDeletedMetric: a successful sweep moves objects_deleted 0 to 1, records a sweep, and leaves deletion_errors at 0.
• TestGarbageCollectorDeletionErrorMetric: an injected delete failure moves deletion_errors 0 to 1, and objects_deleted stays 0.

Verified via the default registry, since endpoint exposure depends on #7453.

go test ./executor/pkg/controller/ -run TestGarbageCollector -count=1 -v

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• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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[Sovietaced]

Is it possible to rework these using OTEL instead of prometheus?

[davidlin20dev]

Is it possible to rework these using OTEL instead of prometheus?

Hi @Sovietaced, happy to switch these to OTel.

I went with Prometheus because #7445 and #7455 (umbrella issue) are both centered around promutils, so that's
the direction I followed. Before I rework it, though, I just want to make sure I'm building the right thing.

@pingsutw, since you opened these issues: could you confirm whether OTel is the preferred
direction now, and whether the issues should be updated to reflect that? Happy to rework this
on the meter provider (mirroring #7483) once I know which way to go.

Thanks both, appreciate the help!