Expose Per-Namespace Pending Task Metrics for Queue Move Groups

[exodus2512]

What changed

Adds a queue_pending_tasks_per_namespace metric to make
per-namespace pending task counts observable for queue move groups.

The metric is recorded from the existing pending-task calculation in
actionMoveGroup, avoiding any additional task-count computation.

The metric includes:

• namespace
• task_category

Zero pending-task counts are not emitted.

Why

The move-group mechanism uses a pending-task threshold to move noisy
task groups to dedicated readers. Previously, operators could observe
the threshold being exceeded through logs, but could not monitor the
per-namespace pending task count leading up to that threshold.

This metric makes that state observable.

Implementation

• Added QueuePendingTasksPerNamespace dimensionless histogram.
• Recorded the existing pendingTaskPerGroup values.
• Added namespace name resolution with a safe namespace-ID fallback.
• Added support for standard and outbound grouper keys.
• Added focused unit tests for metric emission, tags, zero counts,
  namespace resolution fallback, and outbound grouper keys.

Tests

• go test ./service/history/queues
• go test ./common/metrics
• Focused move-group and checkpoint tests
• git diff --check
• gofmt

All tests pass.

Fixes #12450

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.