The engine's metrics, in Prometheus text exposition format: the Rust crate's trellis::Metrics::render_prometheus (docs/decisions/0009-observability-decisions.md).

The registry is process-wide: one body holds the series of every Trellis handle in this OS process, so it needs no handle and works before Trellis.connect/1 and after Trellis.shutdown/1. Every series carries a trellis_instance="<database>/<catalog schema>" label, the same value as the trellis_instance of the instance's log lines, so handles in one VM stay apart. trellis_instance_up is 1 while an instance has a running handle and 0 after its last one shuts down; the other gauges of a stopped instance disappear after 60 seconds. See docs/observability.md.

The binding opens no port. Serve the text from a route the host already has, for example with Plug:

get "/metrics" do
  conn
  |> put_resp_content_type("text/plain; version=0.0.4")
  |> send_resp(200, Trellis.Metrics.render_prometheus())
end