## Summary
The `runqueue.workerQueue.length` gauge only reported a worker queue's
depth while runs were being dequeued from it. When dequeues stop, the
metric goes stale or missing, so a queue that has backed up because
nothing is draining it can't be alerted on. This adds a small observer
that refreshes the observed set of worker queues from the
`WorkerInstanceGroup` records on an interval, so every active worker
queue (and its scheduled split variant) keeps reporting its length
regardless of dequeue activity.
The observer is off by default and enabled per service via
`RUN_ENGINE_WORKER_QUEUE_OBSERVER_ENABLED`, reads from the read replica,
and skips a configurable set of cloud providers
(`RUN_ENGINE_WORKER_QUEUE_OBSERVER_EXCLUDED_CLOUD_PROVIDERS`, default
`digitalocean`). When enabled it is the source of truth for the observed
set, so the per-dequeue registration is skipped on that instance, and it
groups by worker queue so the per-instance duplicates collapse to the
true depth.
Also removes the unused `GET`/`POST /api/v1/workers` endpoints. Their
only consumer was a CLI command group that is no longer registered.
## Verification
Verified end to end against a local stack: the gauge reports each worker
queue's length with no dequeues happening, excludes the configured
providers, includes hidden groups, and the removed endpoints return as
if they never existed. Added a run-engine test
(`workerQueueObservation.test.ts`).
Eric Allam
8890d7a258