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>

README.md

@@ -71,6 +71,11 @@ error: www.adbraun.lol has a CNAME, so it can't have any other records (found A)
   queries, record types, cache hit rate. Shown in the web UI and the CLI,
   and exportable as JSON. Stored as aggregates in a separate database with
   retention you set.
+* **Prometheus metrics** (optional): health to alert on (named down,
+  configuration problems, drift, failing blocklists, stale backups, DNSSEC
+  steps waiting at the parent, expiring tokens) and query counters per
+  result code, view, zone and blocklist, with example alerts. Set up in
+  jarl.yaml, with an address allow list and/or a bearer token.
 * **Blocking** like Pi-hole, done by named itself: blocklists (Adblock,
   hosts or plain domains) downloaded and kept up to date, your own block
   and allow rules that beat every list, per view, with "why is this
@@ -120,6 +125,7 @@ in `/etc/jarl/jarl.yaml` to reach it from your management network.
 | [docs/dnssec.md](docs/dnssec.md) | DNSSEC signing, going public with your own name servers, Porkbun |
 | [docs/acme.md](docs/acme.md) | Let's Encrypt DNS-01 with certbot, acme.sh, lego and Traefik |
 | [docs/stats.md](docs/stats.md) | Query statistics: enabling, what is collected, retention |
+| [docs/metrics.md](docs/metrics.md) | Prometheus metrics: access, what is exported, example alerts |
 | [docs/blocking.md](docs/blocking.md) | Blocking ads, trackers and malware: blocklists, your own rules, views, memory |
 | [docs/validation.md](docs/validation.md) | Every check Jarl makes |
 | [docs/operations.md](docs/operations.md) | Backups, drift, failures, recovery, monitoring |