[flyte2] Executor: expose promutils (default-registry) metrics on the controller-runtime metrics endpoint

> Part of #7445.

## Summary

The executor exposes a Prometheus `/metrics` endpoint via the controller-runtime metrics server, but it serves **controller-runtime's own registry** — so metrics registered through `promutils` (which use the **default** Prometheus registry) are collected but never scraped. Bridge the two so the `executor:*` scoped metrics actually show up.

## Background

`executor/setup.go:75-91` configures the controller-runtime metrics server:

```go
metricsServerOptions := metricsserver.Options{ BindAddress: cfg.MetricsBindAddress, ... } // default ":10254" (config.go:15)
mgr, err := ctrl.NewManager(sc.K8sConfig, ctrl.Options{ Metrics: metricsServerOptions, ... })
```

The controller-runtime metrics server serves `sigs.k8s.io/controller-runtime/pkg/metrics`'s **own** registry (controller-runtime built-ins + Go/process collectors).

Meanwhile `promutils` registers every metric on the **default** Prometheus registry — see `flytestdlib/promutils/scope.go:269-419`, all using `prometheus.Register(...)` (i.e. `prometheus.DefaultRegisterer`). The executor wires several such scopes:

- `promutils.NewScope("executor")` for the webhook (`setup.go:112`)
- `promutils.NewScope("executor:storage")` for the data store (`setup.go:116`)
- `promutils.NewScope("executor")` for plugins (`setup.go:124`)
- `promutils.NewScope("executor:catalog")` for the catalog client (`setup.go:142`)

Nothing bridges the default registry into the controller-runtime registry, so **none of these `executor:*` metrics are exposed** on `:10254`. They're dead today.

## What to do

Make the default-registry metrics scrapeable on the metrics endpoint. Options (pick the cleanest; discuss in the issue):

1. **Register the default gatherer with controller-runtime's registry** — e.g. have controller-runtime's metrics server also gather `prometheus.DefaultGatherer`, or register the relevant collectors into `ctrlmetrics.Registry`.
2. **Point promutils at controller-runtime's registry** — less ideal, would require a promutils change.
3. **Serve a merged handler** — expose a handler that gathers from both registries.

## Acceptance criteria

- [ ] Scraping the executor metrics endpoint (`:10254` by default) returns both controller-runtime built-ins **and** the `executor:*` / `executor_*` metrics registered via promutils (storage, catalog, webhook).
- [ ] No duplicate-registration panics at startup.
- [ ] **Verification:** the PR description shows a `curl /metrics` excerpt (or a test) demonstrating an `executor`-scoped metric appears.

## Pointers

- `executor/setup.go:75-142` — metrics server options, manager creation, and the promutils scopes.
- `flytestdlib/promutils/scope.go:269-419` — confirms promutils uses `prometheus.Register` (default registry).
- controller-runtime metrics docs: https://book.kubebuilder.io/reference/metrics

## Notes for contributors

- This is the highest-impact executor metrics fix: the metrics already exist, they're just not exposed.
- Coordinate with #7446 (which adds `/metrics` to the non-controller services) for naming consistency, but this issue is executor-specific and can proceed independently.