feat: Add metrics

Fixes #7.

Signed-off-by: Julien Riou <julien@riou.xyz>

src/server/metrics.go

@@ -0,0 +1,55 @@
+package server
+
+import (
+	"fmt"
+	"log/slog"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type Metrics struct {
+	logger   *slog.Logger
+	notes    prometheus.Gauge
+	db       *Database
+	reg      *prometheus.Registry
+	interval int
+}
+
+func NewMetrics(logger *slog.Logger, reg *prometheus.Registry, config *Config, db *Database) (*Metrics, error) {
+	l := logger.With("module", "metrics")
+	m := &Metrics{
+		logger: l,
+		db:     db,
+		notes: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: config.PrometheusNotesMetric,
+		}),
+		reg:      reg,
+		interval: config.ObservationInterval,
+	}
+	if err := reg.Register(m.notes); err != nil {
+		return nil, err
+	}
+
+	go m.ObserveThread()
+
+	return m, nil
+}
+
+func (m *Metrics) ObserveThread() {
+	m.logger.Debug("starting observations")
+	for {
+		m.logger.Debug("counting notes")
+		var count int64
+		m.db.db.Model(&Note{}).Count(&count)
+		m.notes.Set(float64(count))
+		m.logger.Debug("counted notes", slog.Any("count", count))
+
+		wording := "second"
+		if m.interval > 1 {
+			wording += "s"
+		}
+		m.logger.Debug(fmt.Sprintf("waiting for %d %s before next observation", m.interval, wording))
+		time.Sleep(time.Duration(m.interval) * time.Second)
+	}
+}