[flyte2] Emit Prometheus metrics from v2 services (runs service + app framework)

## Summary

The v2 services built on the `flytestdlib/app` framework (starting with the **runs service**) currently expose **no Prometheus metrics**. This is an umbrella issue tracking the work to add observability, broken into small, independent tasks that are great for first-time contributors.

## Background

When you run the runs service (`runs/cmd/main.go` → `app.App.Run()`), the only HTTP endpoints served are `/healthz` and `/readyz` (`flytestdlib/app/app.go:94-109`). There is **no `/metrics` endpoint**, and nothing emits Prometheus metrics:

- `app.SetupContext` has a `Scope promutils.Scope` field (`flytestdlib/app/context.go:62-63`), but it is **never assigned**.
- `runs/cmd/main.go:43` passes `promutils.NewTestScope()` into the `DataStore` — a throwaway scope that registers nothing.
- `runs/setup.go` registers no counters/gauges/timers and no RPC interceptors.
- The old v1 pattern that served `/metrics` (`flytestdlib/profutils/server.go:118`, `promhttp.Handler()`) was **not ported** into the v2 `app` framework.

So today a Prometheus scrape of the runs service returns nothing useful.

## Goal

Give every v2 service that uses the `app` framework a Prometheus `/metrics` endpoint, a real metrics `Scope`, and meaningful instrumentation on the runs service hot paths.

## Tasks

This work is split so multiple contributors can pick up pieces in parallel. **Task 1 is the foundation and must land first**; tasks 2–4 depend on it and can then proceed independently.

- [ ] #7446 (**foundation**): Add a `/metrics` endpoint and initialize the metrics `Scope` in the `app` framework — _blocker for the rest_
- [ ] #7447: Add gRPC/Connect RPC metrics interceptor to the runs service _(depends on #7446)_
- [ ] #7448: Instrument the runs DB repository layer with Prometheus metrics _(depends on #7446)_
- [ ] #7449: Instrument the runs reconcilers (abort-reconciler) with Prometheus metrics _(depends on #7446)_
- [ ] #7450: Instrument the actions service (watcher metrics + dropped-updates counter) _(depends on #7446)_

**Executor** (controller-runtime based — already has a `/metrics` endpoint, but gaps remain):
- [ ] #7453: Expose promutils (default-registry) metrics on the controller-runtime metrics endpoint — _scoped metrics are collected but not scraped_
- [ ] #7455: Add custom domain metrics (TaskAction reconcile, cache, GC, plugins)
- [ ] #7456: Dedupe the two `promutils.NewScope("executor")` constructions in setup.go

## How to claim a task

Comment on the specific sub-issue you'd like to work on and a maintainer will assign it. Please land the foundation issue (#7446) first, since the others build on it. Each sub-issue is self-contained with file references and acceptance criteria.