[flyte2] Executor: dedupe the two promutils.NewScope("executor") constructions in setup.go

Part of #7445. Small starter cleanup.

Summary

executor/setup.go constructs promutils.NewScope("executor") twice with the same base name. Dedupe it into a single shared scope to avoid confusion and the risk of duplicate-metric-registration panics.

Background

In executor/setup.go:

• setup.go:112 — webhookPkg.Setup(ctx, kubeClient, wCfg, podNamespace, promutils.NewScope("executor"), mgr)
• setup.go:124 — plugin.NewSetupContext(mgr, nil, nil, nil, nil, "TaskAction", promutils.NewScope("executor"))

Both create a scope with the identical base name "executor". promutils registers metrics on the default Prometheus registry via prometheus.Register(...) (flytestdlib/promutils/scope.go:269-419), which panics on duplicate registration. Today this is latent (the two consumers happen to register different metric names), but two independent scopes sharing a base name is fragile — if both ever register a like-named metric, the executor panics at startup.

What to do

1. Construct the executor base scope once, e.g.:

   executorScope := promutils.NewScope("executor")
   

2. Pass distinct sub-scopes to each consumer so names can't collide, e.g.:
   • webhook: executorScope.NewSubScope("webhook")
   • plugins: executorScope.NewSubScope("plugin") (Adjust names to match conventions; the storage and catalog scopes already use distinct names executor:storage / executor:catalog.)

Acceptance criteria

• promutils.NewScope("executor") is constructed exactly once in executor/setup.go.
• Webhook and plugin consumers receive distinct sub-scopes derived from it.
• The executor still starts cleanly (no duplicate-registration panic) and existing tests pass.

Pointers

• executor/setup.go:112 and setup.go:124 — the two NewScope("executor") calls.
• flytestdlib/promutils/scope.go — NewScope / NewSubScope and the prometheus.Register calls that panic on duplicates.

Notes for contributors

• Pure refactor — no behavior change beyond scope naming. Great first PR.