feat: add Prometheus metrics (#439)

* feat: add Prometheus metrics

* setup metrics using Prometheus client under /metrics endpoint
* add configuration value for metrics
* configure Prometheus middleware for generic metrics
* provide metrics for totals of users, gists and SSH keys
* modify test request to optionally return the response
* provide integration test for Prometheus metrics
* update documentation

* chore: make fmt

internal/web/handlers/health/metrics.go

@@ -1,9 +0,0 @@
-package health
-
-import "github.com/thomiceli/opengist/internal/web/context"
-
-// Metrics is a dummy handler to satisfy the /metrics endpoint (for Prometheus, Openmetrics, etc.)
-// until we have a proper metrics endpoint
-func Metrics(ctx *context.Context) error {
-	return ctx.String(200, "")
-}

internal/web/handlers/metrics/metrics.go

@@ -0,0 +1,101 @@
+package metrics
+
+import (
+	"github.com/labstack/echo-contrib/echoprometheus"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/thomiceli/opengist/internal/config"
+	"github.com/thomiceli/opengist/internal/db"
+	"github.com/thomiceli/opengist/internal/web/context"
+)
+
+var (
+	// Using promauto to automatically register metrics with the default registry
+	countUsersGauge   prometheus.Gauge
+	countGistsGauge   prometheus.Gauge
+	countSSHKeysGauge prometheus.Gauge
+
+	metricsInitialized bool = false
+)
+
+// initMetrics initializes metrics if they're not already initialized
+func initMetrics() {
+	if metricsInitialized {
+		return
+	}
+
+	// Only initialize metrics if they're enabled
+	if config.C.MetricsEnabled {
+		countUsersGauge = promauto.NewGauge(
+			prometheus.GaugeOpts{
+				Name: "opengist_users_total",
+				Help: "Total number of users",
+			},
+		)
+
+		countGistsGauge = promauto.NewGauge(
+			prometheus.GaugeOpts{
+				Name: "opengist_gists_total",
+				Help: "Total number of gists",
+			},
+		)
+
+		countSSHKeysGauge = promauto.NewGauge(
+			prometheus.GaugeOpts{
+				Name: "opengist_ssh_keys_total",
+				Help: "Total number of SSH keys",
+			},
+		)
+
+		metricsInitialized = true
+	}
+}
+
+// updateMetrics refreshes all metric values from the database
+func updateMetrics() {
+	// Only update metrics if they're enabled
+	if !config.C.MetricsEnabled || !metricsInitialized {
+		return
+	}
+
+	// Update users count
+	countUsers, err := db.CountAll(&db.User{})
+	if err == nil {
+		countUsersGauge.Set(float64(countUsers))
+	}
+
+	// Update gists count
+	countGists, err := db.CountAll(&db.Gist{})
+	if err == nil {
+		countGistsGauge.Set(float64(countGists))
+	}
+
+	// Update SSH keys count
+	countKeys, err := db.CountAll(&db.SSHKey{})
+	if err == nil {
+		countSSHKeysGauge.Set(float64(countKeys))
+	}
+}
+
+// Metrics handles prometheus metrics endpoint requests.
+func Metrics(ctx *context.Context) error {
+	// If metrics are disabled, return 404
+	if !config.C.MetricsEnabled {
+		return ctx.NotFound("Metrics endpoint is disabled")
+	}
+
+	// Initialize metrics if not already done
+	initMetrics()
+
+	// Update metrics
+	updateMetrics()
+
+	// Get the Echo context
+	echoCtx := ctx.Context
+
+	// Use the Prometheus metrics handler
+	handler := echoprometheus.NewHandler()
+
+	// Call the handler
+	return handler(echoCtx)
+}