code: func requestMetrics(l *log.Logger) service {
  	return func(h http.Handler) http.Handler {
  		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  			start := time.Now()
  			h.ServeHTTP(w, r)
  			l.Printf("%s request to %s took %vns.", r.Method, r.URL.Path, time.Now().Sub(start).Nanoseconds())
  		})
  	}
  }
docs: // middleware for request length metrics.