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/api.md

@@ -10,6 +10,8 @@ what is documented here.
   (`www.adbraun.lol.`), and display helpers drop it (`display_name`,
   `relative_name`).
 * `GET /healthz` (no authentication) returns `{"ok": true, "version": ...}`.
+* `GET /metrics` serves Prometheus metrics when `metrics.enabled` is set.
+  Access is set in jarl.yaml, not with API tokens ([metrics.md](metrics.md)).
 
 ## Authentication