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>

internal/server/server.go

@@ -58,6 +58,8 @@ type Config struct {
 	// name authenticator apps show (h_totp.go).
 	TOTPRequire string
 	TOTPIssuer  string
+	// Metrics configures /metrics (metrics.go).
+	Metrics Metrics
 }
 
 // Server is the HTTP server.
@@ -80,6 +82,10 @@ type Server struct {
 	// nsChecker, when set, replaces the default name server checker
 	// (tests point it at local DNS servers).
 	nsChecker *nscheck.Checker
+	// started is when the server was created; metricsCache holds the Jarl
+	// state part of /metrics.
+	started      time.Time
+	metricsCache metricsCache
 
 	mu   sync.Mutex
 	srvs []*http.Server
@@ -108,7 +114,7 @@ func New(cfg Config, eng *engine.Engine, log *slog.Logger, web fs.FS) *Server {
 	if log == nil {
 		log = slog.Default()
 	}
-	s := &Server{cfg: cfg, eng: eng, store: eng.Store(), log: log, mux: http.NewServeMux(), limiter: newRateLimiter(), web: web}
+	s := &Server{cfg: cfg, eng: eng, store: eng.Store(), log: log, mux: http.NewServeMux(), limiter: newRateLimiter(), web: web, started: time.Now()}
 	s.expiryWarn.every = 24 * time.Hour
 	if cfg.Stats != nil {
 		s.statsQ = stats.NewQuerier(cfg.Stats)
@@ -200,6 +206,10 @@ func (s *Server) Serve(ctx context.Context, ready func()) error {
 			}
 		}
 	}
+	if err := s.serveMetricsListeners(errc); err != nil {
+		s.shutdown()
+		return err
+	}
 	s.serveSocket(errc)
 	s.logHTTPReq(tlsCfg != nil)
 	if ready != nil {