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/metrics/collector/list.go

@@ -0,0 +1,78 @@
+// Copyright (c) 2021 - 2025, Ludvig Lundgren and the autobrr contributors.
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package collector
+
+import (
+	"context"
+
+	"github.com/autobrr/autobrr/internal/list"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type listCollector struct {
+	listService list.Service
+
+	totalCount           *prometheus.Desc
+	enabledCount         *prometheus.Desc
+	LastRefreshTimestamp *prometheus.Desc
+	errorMetric          *prometheus.Desc
+}
+
+func (collector *listCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- collector.totalCount
+	ch <- collector.enabledCount
+	ch <- collector.LastRefreshTimestamp
+	ch <- collector.errorMetric
+}
+
+func (collector *listCollector) Collect(ch chan<- prometheus.Metric) {
+	lists, err := collector.listService.List(context.TODO())
+	if err != nil {
+		ch <- prometheus.NewInvalidMetric(collector.errorMetric, err)
+		return
+	}
+
+	enabled := 0
+	for _, l := range lists {
+		if l.Enabled {
+			enabled++
+		}
+
+		if !l.LastRefreshTime.IsZero() {
+			ch <- prometheus.MustNewConstMetric(collector.LastRefreshTimestamp, prometheus.GaugeValue, float64(int(l.LastRefreshTime.Unix())), l.Name)
+		}
+	}
+	ch <- prometheus.MustNewConstMetric(collector.totalCount, prometheus.GaugeValue, float64(len(lists)))
+	ch <- prometheus.MustNewConstMetric(collector.enabledCount, prometheus.GaugeValue, float64(enabled))
+}
+
+func NewListCollector(listService list.Service) *listCollector {
+	return &listCollector{
+		listService: listService,
+		totalCount: prometheus.NewDesc(
+			"autobrr_list_total",
+			"Number of lists",
+			nil,
+			nil,
+		),
+		enabledCount: prometheus.NewDesc(
+			"autobrr_list_enabled_total",
+			"Number of enabled lists",
+			nil,
+			nil,
+		),
+		LastRefreshTimestamp: prometheus.NewDesc(
+			"autobrr_list_last_refresh_timestamp_seconds",
+			"The timestamp of the last list run",
+			[]string{"list"},
+			nil,
+		),
+		errorMetric: prometheus.NewDesc(
+			"autobrr_list_collector_error",
+			"Error while collecting list metrics",
+			nil,
+			nil,
+		),
+	}
+}