Extract shared ASB Prometheus helpers

[haasonsaas]

Summary

• extract shared Prometheus metric registration helpers into internal/promutil
• switch internal/app and internal/worker metrics to the shared helper package
• remove the duplicated local implementations without changing metric names or registration behavior

Testing

• go test ./internal/app ./internal/worker -count=1
• go test ./... -count=1

[cursor]

PR Summary

Low Risk
Low risk refactor that centralizes metric prefix normalization and collector registration logic; behavior should remain the same but could impact metric naming/registration if the shared helpers diverge.

Overview
Extracts shared Prometheus utilities into new internal/promutil (MetricPrefix plus Register* helpers that reuse already-registered collectors).

Updates both internal/app and internal/worker metrics initialization to use promutil and removes their duplicated local implementations, without changing the metric definitions themselves.

^{Reviewed by Cursor Bugbot for commit 54af392. Bugbot is set up for automated code reviews on this repo. Configure here.}