[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:

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 <metricsBindAddress>/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.