P2: Shared packages — Prometheus metric leak, event listener panic risk, status code bug #689

Summary

pkgs/lockmetrics — Metric Leak

• Prometheus metrics never unregistered: Each RWMutex creates metrics; Close() only removes from tracking map. Long-running apps with dynamic resources leak thousands of metric instances.
• WithLabelValues() hot-loop overhead: Hash lookup on every lock/unlock.

pkgs/handler — Status Code Bug

• ResponseWriter: statusCode initialized to 200, making the if statusCode == 0 check in Write() dead code.

pkgs/events — Listener Panic Risk

• No panic recovery: If a listener panics, the entire Emit() crashes.
• Inefficient unsubscribe: removeByID uses append([:i], [i+1:]...).

Files

• pkgs/lockmetrics/lockmetrics.go
• pkgs/handler/response_writer.go
• pkgs/events/emitter.go