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>

cmd/jarld/main.go

@@ -303,6 +303,11 @@ func serveCmd() *cobra.Command {
 					a.log.Warn("the configuration file holds the Porkbun API key and everyone can read it; make it readable only by root and jarld (chown root:jarl, chmod 640)", "file", configPath)
 				}
 			}
+			if a.cfg.Metrics.Enabled && a.cfg.Metrics.Token != "" {
+				if fi, err := os.Stat(configPath); err == nil && fi.Mode().Perm()&0o004 != 0 {
+					a.log.Warn("the configuration file holds metrics.token and everyone can read it; make it readable only by root and jarld (chown root:jarl, chmod 640)", "file", configPath)
+				}
+			}
 			srv := server.New(server.Config{
 				Listen: a.cfg.Listen, TLSCertFile: a.cfg.TLS.CertFile, TLSKeyFile: a.cfg.TLS.KeyFile,
 				TrustedProxies: a.cfg.Proxies(), SessionLifetime: time.Duration(a.cfg.Session.Lifetime),
@@ -311,6 +316,8 @@ func serveCmd() *cobra.Command {
 				BackupDir: a.cfg.Backup.Dir, BackupWarnAfter: time.Duration(a.cfg.Backup.WarnAfter),
 				Porkbun: pb, PorkbunAllows: a.cfg.Porkbun.Allows,
 				TOTPRequire: a.cfg.TOTP.Require, TOTPIssuer: a.cfg.TOTP.IssuerName(),
+				Metrics: server.Metrics{Enabled: a.cfg.Metrics.Enabled, Listen: a.cfg.Metrics.Listen,
+					Allow: a.cfg.Metrics.AllowPrefixes(), Token: a.cfg.Metrics.Token},
 				HTTPReq: server.HTTPReq{
 					Enabled:           a.cfg.ACME.HTTPReq.Enabled,
 					AllowPlainHTTP:    !a.cfg.ACME.HTTPReq.RequireHTTPS,