[flyte2] Instrument the runs service DB repository layer with OTEL metrics

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

## Summary

Add Prometheus metrics to the runs service database repository layer so we can observe DB call volume, error rates, and latency per operation.

## Background

The repository implementations live in `runs/repository/impl/` (e.g. `task.go`, `trigger.go`, and the run repo). These wrap GORM/DB calls and currently emit **no metrics** — there is no visibility into how often each query runs, how long it takes, or how often it fails.

## What to do

1. Thread the metrics `Scope` (from #7446, available via `app.SetupContext.Scope`) into the repository constructor(s). `runs/setup.go` calls `repository.NewRepository(sc.DB, cfg.Database)` and `impl.NewProjectRepo(sc.DB)` — extend these (or wrap the repo) to accept a `promutils.Scope`.
2. For each DB operation (create/get/list/update/delete), record:
- call count (labeled by operation name)
- error count (labeled by operation name)
- latency (a Prometheus timer / stopwatch)

A small helper that wraps a DB call with `start := time.Now(); defer timer.Stop()` + counter increments keeps this DRY.

## Acceptance criteria

- [ ] `/metrics` exposes per-operation DB call count, error count, and latency for the runs repository.
- [ ] Metrics are created once (no duplicate-registration panics) using a dedicated sub-scope, e.g. `scope.NewSubScope("db")`.
- [ ] Unit tests assert that a repository operation increments the expected counter / records latency.

## Pointers

- `runs/repository/impl/` — repository implementations to instrument.
- `runs/repository/repository.go` (the `NewRepository` constructor) and `runs/setup.go:40` where it's called.
- `flytestdlib/promutils/scope.go` — `Scope` helpers (`MustNewCounter`, `MustNewStopWatch`, `NewSubScope`).

## Notes for contributors

- Keep label cardinality low: label by **operation name**, never by row IDs / project / user values.
- This can be done independently of #7447 (RPC interceptors); both consume the same `Scope`.