feat: add prometheus metrics

backend/db/metrics.go

@@ -0,0 +1,63 @@
+package db
+
+import (
+	"context"
+
+	"codeberg.org/u1f320/pronouns.cc/backend/log"
+	"emperror.dev/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+func (db *DB) initMetrics() (err error) {
+	err = prometheus.Register(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "pronouns_users_total",
+		Help: "The total number of registered users",
+	}, func() float64 {
+		count, err := db.TotalUserCount(context.Background())
+		if err != nil {
+			log.Errorf("getting user count for metrics: %v", err)
+		}
+		return float64(count)
+	}))
+	if err != nil {
+		return errors.Wrap(err, "registering user count gauge")
+	}
+
+	err = prometheus.Register(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "pronouns_members_total",
+		Help: "The total number of registered members",
+	}, func() float64 {
+		count, err := db.TotalMemberCount(context.Background())
+		if err != nil {
+			log.Errorf("getting member count for metrics: %v", err)
+		}
+		return float64(count)
+	}))
+	if err != nil {
+		return errors.Wrap(err, "registering member count gauge")
+	}
+
+	db.TotalRequests = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "pronouns_api_requests_total",
+		Help: "The total number of API requests since the last restart",
+	})
+
+	return nil
+}
+
+func (db *DB) TotalUserCount(ctx context.Context) (numUsers int64, err error) {
+	err = db.QueryRow(ctx, "SELECT COUNT(*) FROM users WHERE deleted_at IS NULL").Scan(&numUsers)
+	if err != nil {
+		return 0, errors.Wrap(err, "querying user count")
+	}
+	return numUsers, nil
+}
+
+func (db *DB) TotalMemberCount(ctx context.Context) (numMembers int64, err error) {
+	err = db.QueryRow(ctx, "SELECT COUNT(*) FROM members WHERE unlisted = false AND user_id = ANY(SELECT id FROM users WHERE deleted_at IS NULL)").Scan(&numMembers)
+	if err != nil {
+		return 0, errors.Wrap(err, "querying member count")
+	}
+	return numMembers, nil
+}