Function metrics_handler 

pub async fn metrics_handler(__arg0: State<Arc<Metrics>>) -> Response

GET /metrics — Prometheus text exposition (text/plain; version=0.0.4).

Authentication

Intentionally unauthenticated. Matches Prometheus operational convention: network-layer (ingress ACL, service-mesh policy, or cluster-internal-only listen) gates scrape access. FlowFabric does not own auth for scrape endpoints.

If you need to restrict scrapers, constrain the listen address (bind to the metrics-only interface) or set ingress rules.