Prometheus metrics at /metrics, configured in jarl.yaml

New metrics: section (enabled, listen, allow, token). /metrics is served on
the main listeners, the local socket (root and jarld's user) and optional
metrics-only addresses; a scrape needs an allowed address or the bearer
token. Exports health (named up, problems, drift, backups, blocklists,
DNSSEC phases, token expiry) and, with stats on, named's and Jarl's query
counters since jarld started plus named's process and cache gauges.
docs/metrics.md has example queries and alerts checked with promtool.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/configuration.md

@@ -73,6 +73,18 @@ The `stats:` section turns on query statistics (off by default). See
 [stats.md](stats.md) for every setting and what it changes in named's
 configuration. Changing it takes effect when jarld restarts.
 
+### Prometheus metrics
+
+| Key | Default | Meaning |
+|-----|---------|---------|
+| `metrics.enabled` | `false` | Serve Prometheus metrics at `/metrics` on `listen` and the local socket ([metrics.md](metrics.md)). |
+| `metrics.listen` | `[]` | Extra `ADDRESS:PORT`s that serve only `/metrics`, over plain HTTP, e.g. `["0.0.0.0:9450"]`. |
+| `metrics.allow` | `["127.0.0.1", "::1"]` | Addresses or prefixes that may scrape without a token. |
+| `metrics.token` | `""` | A bearer token (at least 16 characters) that lets any address scrape. Keep jarl.yaml readable by root and jarld only when set. |
+
+Query counters need `stats.enabled` as well. Changes take effect when jarld
+restarts.
+
 ### ACME (lego httpreq endpoint)
 
 An endpoint for lego's `httpreq` DNS provider, so lego and Traefik can