feat: add prometheus metrics

hello/metrics.go

@@ -0,0 +1,52 @@
+package hello
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	httpRequestsTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Name: "http_requests_total",
+		Help: "Total number of HTTP requests.",
+	}, []string{"path", "method", "status"})
+
+	httpRequestDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Name:    "http_request_duration_seconds",
+		Help:    "Duration of HTTP requests.",
+		Buckets: prometheus.DefBuckets,
+	}, []string{"path", "method", "status"})
+)
+
+// Middleware wraps an http.HandlerFunc with Prometheus metrics
+func Middleware(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		start := time.Now()
+
+		ww := &responseWriter{ResponseWriter: w, status: http.StatusOK}
+
+		next(ww, r)
+
+		duration := time.Since(start).Seconds()
+		status := strconv.Itoa(ww.status)
+		path := r.URL.Path
+		method := r.Method
+
+		httpRequestsTotal.WithLabelValues(path, method, status).Inc()
+		httpRequestDuration.WithLabelValues(path, method, status).Observe(duration)
+	}
+}
+
+type responseWriter struct {
+	http.ResponseWriter
+	status int
+}
+
+func (rw *responseWriter) WriteHeader(code int) {
+	rw.status = code
+	rw.ResponseWriter.WriteHeader(code)
+}