[AMORO-4313] AIP-5 Phase 5: observability metrics for dynamic allocation

[j1wonpark]

Why are the changes needed?

Close #4313.

Final phase of AIP-5. The scaling behavior from #4272 and #4296 is currently observable only through INFO logs; this exposes it as metrics so operators can alert on it — in particular resource thrashing ("N scale-downs in the last hour", increase(optimizer_group_scale_down_total[1h])) and the config fail-safe fallback that would otherwise be silent.

Brief change log

New per-group metrics (all tagged group), registered in two places by ownership of their source state:

• OptimizerGroupMetrics (every group, queue lifecycle):
  • optimizer_group_idle_optimizers (gauge) — instances with zero in-flight (SCHEDULED/ACKED) tasks
  • optimizer_group_config_invalid (gauge) — 1 while an opted-in group's DRA config is invalid. Registered for every group because an invalid config makes the group not effectively enabled — tying this gauge to the DRA watch would remove it exactly when it should alarm.
• New DynamicAllocationMetrics (DRA groups only, scale-keeper watch lifecycle):
  • optimizer_group_pending_removal_optimizers (gauge) — instances in graceful drain
  • optimizer_group_effective_threads (gauge) — registered threads + threads of optimizers pending registration, the raw observable as defined in the AIP (the keeper's round arithmetic additionally subtracts draining instances internally)
  • optimizer_group_backlog_duration_ms (gauge) — duration since demand first exceeded capacity, derived at scrape time (keeper rounds are seconds apart, pushing would go stale between scrapes)
  • optimizer_group_scale_up_total / optimizer_group_scale_down_total (counters) — one attempted scale-out round (regardless of instance count or request outcome) and one drain start each count 1; instance-count trends are already covered by optimizer_group_optimizer_instances, and a rising counter without rising instances sustained beyond the pod boot window signals failing resource requests

Naming follows the Prometheus/OpenMetrics conventions (_total counter suffix, _ms unit suffix) as published in the AIP, rather than the pre-reporter _count/_mills suffixes of the existing table metrics.

Metric lifecycle is hardened against the failure paths it introduces: registration failures roll back partially registered metrics so the group stays rewatchable, the keeper's dispose unregisters its metrics (the global registry outlives the service across HA leader hand-offs), and a watch arriving after dispose is a no-op (stopped guard, the same pattern as OptimizerGroupKeeper.keepInTouch). Watch/unwatch are serialized, and a disable unwatches on the config-entry path itself — the round-driven unwatch runs on the leader only, so followers would otherwise keep exporting the group's DRA metrics until failover.

Supporting changes: DynamicAllocationState.backlogDurationMs() read accessor, DynamicAllocationConfig.isConfigInvalid() (a group that never opted in is never invalid, whatever its leftover properties parse to), and the metrics table in docs/user-guides/metrics.md.

Known trade-offs

• Disabling and re-enabling dynamic allocation re-registers the keeper-scoped metrics, resetting the counters to 0. Prometheus increase()/rate() are counter-reset tolerant, so alerts survive this.
• On HA standby nodes the gauges are registered (as the existing group metrics are) but the counters stay 0, since only the leader evaluates scaling.
• The backlog_duration_ms gauge wiring is covered by unit tests on the accessor plus the keeper lambda; an end-to-end assertion under real demand would need the heavy real-table demand harness already declared out of scope in the previous phases.

Documents & Website changes

docs/user-guides/metrics.md: 2 rows added to the optimizer group table, plus a dynamic-allocation metrics table with the counter semantics.

How was this patch tested?

• TestDynamicAllocationMetrics (new): gauge/counter registration, source-backed values, unregistration, rollback of partial registration on failure.
• TestOptimizerScaleKeeper: metrics registered on watch and removed on disable; a floor round counts one scale-up action; a drain start counts one scale-down action; a failed watch leaves the group rewatchable; dispose unregisters the metrics and a watch arriving after dispose registers nothing; an enabled-to-enabled config update re-enters watch idempotently.
• TestOptimizingQueue: idle-optimizer gauge flips with an in-flight task; config-invalid gauge flips on an invalid runtime update.
• TestDynamicAllocationConfig / TestComputeScaleUp: isConfigInvalid and backlogDurationMs unit coverage.

All new code was written test-first; full DRA suite green (184 tests, Phase 1–4 tests unmodified).