[flyte2] Instrument the runs service reconcilers (abort-reconciler) with Prometheus metrics

> Part of #7445. **Depends on #7446** (the `/metrics` endpoint + `Scope` must exist first).

## Summary

Add Prometheus metrics to the runs service background reconcilers (starting with the **abort reconciler**) to observe queue depth, processing throughput, retries, and failures.

## Background

`runs/service/abort_reconciler.go` runs as a background worker (registered in `runs/setup.go` via `sc.AddWorker("abort-reconciler", ...)`). It has a worker pool, a bounded queue (`QueueSize: 1000`), and retry logic (`MaxAttempts`, `InitialDelay`, `MaxDelay`). None of this is currently observable via metrics.

## What to do

1. Thread the metrics `Scope` (from #7446) into `service.NewAbortReconciler(...)` (extend its config/constructor).
2. Emit metrics such as:
- current queue depth / pending items (gauge)
- items processed (counter, labeled by success/failure)
- retries / attempts (counter)
- per-item processing latency (timer/histogram)

## Acceptance criteria

- [ ] `/metrics` exposes abort-reconciler queue depth, processed count (success/failure), retry count, and processing latency.
- [ ] Metrics use a dedicated sub-scope, e.g. `scope.NewSubScope("abort_reconciler")`, created once.
- [ ] A unit test verifies that processing an item updates the relevant counters/gauges.

## Pointers

- `runs/service/abort_reconciler.go` — the reconciler implementation and its run loop.
- `runs/setup.go:64-73` — where `NewAbortReconciler` is constructed and registered as a worker.
- `flytestdlib/promutils/scope.go` — `Scope` helpers (`MustNewGauge`, `MustNewCounter`, `MustNewStopWatch`, `NewSubScope`).

## Notes for contributors

- The gauge for queue depth should be updated as items are enqueued/dequeued (or sampled periodically).
- This is independent of #7447 and #7448; all three consume the same `Scope` from #7446.