Prometheus metrics at /metrics and pprof at /debug/pprof/ are served on SEIKAN_METRICS_ADDR (default 127.0.0.1:9090) — bound to localhost, so scrape locally or over an SSH tunnel. Key series:

Metric Meaning
seikan_http_requests_total{frontend,method,code} Proxied HTTP requests.
seikan_http_request_duration_seconds{frontend} Per-frontend request latency histogram.
seikan_tunnel_bytes_total{direction} Bytes tunneled (in/out).
seikan_active_tunnels / seikan_tunnel_connects_total Connected tunnels / total connects.
seikan_active_tcp_connections In-flight public TCP connections.
seikan_auth_failures_total{kind} Rejected auth attempts (api_key / token).