A second metrics-enabled router in one process panicked on a duplicate collector registration, because every collector registered on Prometheus's global default registry. metrics.NewRegistry now builds one registry with the Go runtime and process collectors; fx provides it and the delivery metric set built on it. The middleware builds its HTTP recorder once on that registry (NewForTest on a fresh one), the engine and handlers take the metric set from fx, and nothing registers on the global default any more.
/metrics is served from the new registry with the same series names, labels and auth. A test builds two metrics-enabled routers in one process.
Model: opus-5-5
