New feature: prometheus metrics endpoint (#46)

server/prometheus.go

@@ -0,0 +1,48 @@
+package server
+
+import (
+	"context"
+	"errors"
+	"log"
+	"net/http"
+
+	"github.com/dcarrillo/whatismyip/internal/setting"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+type Prometheus struct {
+	server *http.Server
+	ctx    context.Context
+}
+
+func NewPrometheusServer(ctx context.Context) *Prometheus {
+	return &Prometheus{
+		ctx: ctx,
+	}
+}
+
+func (p *Prometheus) Start() {
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.Handler())
+
+	p.server = &http.Server{
+		Addr:         setting.App.PrometheusAddress,
+		Handler:      mux,
+		ReadTimeout:  setting.App.Server.ReadTimeout,
+		WriteTimeout: setting.App.Server.WriteTimeout,
+	}
+
+	log.Printf("Starting Prometheus server listening on %s", setting.App.PrometheusAddress)
+	go func() {
+		if err := p.server.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			log.Fatal(err)
+		}
+	}()
+}
+
+func (p *Prometheus) Stop() {
+	log.Print("Stopping Prometheus server...")
+	if err := p.server.Shutdown(p.ctx); err != nil {
+		log.Printf("Prometheus server forced to shutdown: %s", err)
+	}
+}