synapbus/internal/metrics/middleware.go

package metrics

import (
	"net/http"
	"strconv"
	"time"
)

// responseWriter wraps http.ResponseWriter to capture the status code.
type responseWriter struct {
	http.ResponseWriter
	statusCode int
	written    bool
}

func newResponseWriter(w http.ResponseWriter) *responseWriter {
	return &responseWriter{ResponseWriter: w, statusCode: http.StatusOK}
}

func (rw *responseWriter) WriteHeader(code int) {
	if !rw.written {
		rw.statusCode = code
		rw.written = true
	}
	rw.ResponseWriter.WriteHeader(code)
}

func (rw *responseWriter) Write(b []byte) (int, error) {
	if !rw.written {
		rw.written = true
	}
	return rw.ResponseWriter.Write(b)
}

// Unwrap returns the underlying ResponseWriter, supporting http.Flusher etc.
func (rw *responseWriter) Unwrap() http.ResponseWriter {
	return rw.ResponseWriter
}

// Middleware records HTTP request count and duration for every request.
func Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		start := time.Now()
		wrapped := newResponseWriter(w)

		next.ServeHTTP(wrapped, r)

		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).Observe(duration)
	})
}