Prometheus metrics middleware capturing status + duration

                  package metrics

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

  "github.com/prometheus/client_golang/prometheus"
)

type statusWriter struct {
  http.ResponseWriter
  status int
}

func (w *statusWriter) WriteHeader(code int) {
  w.status = code
  w.ResponseWriter.WriteHeader(code)
}

var (
  dur = prometheus.NewHistogramVec(prometheus.HistogramOpts{
    Name: "http_request_duration_seconds",
    Help: "HTTP request duration in seconds.",
  }, []string{"method", "route"})

  codes = prometheus.NewCounterVec(prometheus.CounterOpts{
    Name: "http_responses_total",
    Help: "HTTP responses by status.",
  }, []string{"method", "route", "status"})
)

func Middleware(route string, next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    sw := &statusWriter{ResponseWriter: w, status: http.StatusOK}
    start := time.Now()
    next.ServeHTTP(sw, r)
    dur.WithLabelValues(r.Method, route).Observe(time.Since(start).Seconds())
    codes.WithLabelValues(r.Method, route, strconv.Itoa(sw.status)).Inc()
  })
}