Add observabilitty and monitoring

internal/observability/metrics_middleware.go

@@ -0,0 +1,62 @@
+package observability
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// MetricsMiddleware creates a middleware that records HTTP metrics.
+func MetricsMiddleware(next http.Handler, registry *prometheus.Registry, _ interface{}) http.Handler {
+	if registry == nil {
+		// If metrics are not enabled, pass through without modification
+		return next
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		start := time.Now()
+
+		// Record request size
+		if r.ContentLength > 0 {
+			httpRequestSize.WithLabelValues(r.Method, r.URL.Path).Observe(float64(r.ContentLength))
+		}
+
+		// Wrap response writer to capture status code and response size
+		wrapped := &metricsResponseWriter{
+			ResponseWriter: w,
+			statusCode:     http.StatusOK,
+			bytesWritten:   0,
+		}
+
+		// Call the next handler
+		next.ServeHTTP(wrapped, r)
+
+		// Record metrics after request completes
+		duration := time.Since(start).Seconds()
+		status := strconv.Itoa(wrapped.statusCode)
+
+		httpRequestsTotal.WithLabelValues(r.Method, r.URL.Path, status).Inc()
+		httpRequestDuration.WithLabelValues(r.Method, r.URL.Path, status).Observe(duration)
+		httpResponseSize.WithLabelValues(r.Method, r.URL.Path).Observe(float64(wrapped.bytesWritten))
+	})
+}
+
+// metricsResponseWriter wraps http.ResponseWriter to capture status code and bytes written.
+type metricsResponseWriter struct {
+	http.ResponseWriter
+	statusCode   int
+	bytesWritten int
+}
+
+func (w *metricsResponseWriter) WriteHeader(statusCode int) {
+	w.statusCode = statusCode
+	w.ResponseWriter.WriteHeader(statusCode)
+}
+
+func (w *metricsResponseWriter) Write(b []byte) (int, error) {
+	n, err := w.ResponseWriter.Write(b)
+	w.bytesWritten += n
+	return n, err
+}