feat(metrics): add metrics server (#1930)

* feat(metrics): add metrics server

* chore: update license headers

* feat(metrics): add optional basic auth

* feat(metrics): add go and process collectors

---------

Co-authored-by: ze0s <43699394+zze0s@users.noreply.github.com>
Co-authored-by: ze0s <ze0s@riseup.net>

internal/http/metrics_server.go

@@ -0,0 +1,96 @@
+// Copyright (c) 2021 - 2025, Ludvig Lundgren and the autobrr contributors.
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package http
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"time"
+
+	"github.com/autobrr/autobrr/internal/config"
+	"github.com/autobrr/autobrr/internal/logger"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"github.com/rs/zerolog"
+)
+
+type metricsManager interface {
+	GetRegistry() *prometheus.Registry
+}
+
+type MetricsServer struct {
+	log zerolog.Logger
+
+	config *config.AppConfig
+
+	version string
+	commit  string
+	date    string
+
+	metricsManager metricsManager
+}
+
+func NewMetricsServer(log logger.Logger, config *config.AppConfig, version string, commit string, date string, metricsManager metricsManager) MetricsServer {
+	return MetricsServer{
+		log:     log.With().Str("module", "http").Logger(),
+		config:  config,
+		version: version,
+		commit:  commit,
+		date:    date,
+
+		metricsManager: metricsManager,
+	}
+}
+
+func (s MetricsServer) Open() error {
+	addr := fmt.Sprintf("%v:%v", s.config.Config.MetricsHost, s.config.Config.MetricsPort)
+
+	var err error
+	for _, proto := range []string{"tcp", "tcp4", "tcp6"} {
+		if err = s.tryToServe(addr, proto); err == nil {
+			break
+		}
+
+		s.log.Error().Err(err).Msgf("Failed to start %s server. Attempted to listen on %s", proto, addr)
+	}
+
+	return err
+}
+
+func (s MetricsServer) tryToServe(addr, protocol string) error {
+	listener, err := net.Listen(protocol, addr)
+	if err != nil {
+		return err
+	}
+
+	s.log.Info().Msgf("Starting Metrics %s server. Listening on %s", protocol, listener.Addr().String())
+
+	server := http.Server{
+		Handler:           s.Handler(),
+		ReadHeaderTimeout: time.Second * 15,
+	}
+
+	return server.Serve(listener)
+}
+
+func (s MetricsServer) Handler() http.Handler {
+	r := chi.NewRouter()
+
+	r.Use(middleware.RequestID)
+	r.Use(middleware.RealIP)
+	r.Use(middleware.Recoverer)
+	r.Use(LoggerMiddleware(&s.log))
+
+	if s.config.Config.MetricsBasicAuthUsers != "" {
+		r.Use(BasicAuth("metrics", s.config.Config.MetricsBasicAuthUsers))
+	}
+
+	r.Get("/metrics", promhttp.HandlerFor(s.metricsManager.GetRegistry(), promhttp.HandlerOpts{}).ServeHTTP)
+
+	return r
+}