code: func Exp(r metrics.Registry) {
  	h := ExpHandler(r)
  	// this would cause a panic:
  	// panic: http: multiple registrations for /debug/vars
  	// http.HandleFunc("/debug/vars", e.expHandler)
  	// haven't found an elegant way, so just use a different endpoint
  	http.Handle("/debug/metrics", h)
  	http.Handle("/debug/metrics/prometheus", prometheus.Handler(r))
  }
docs: // Exp will register an expvar powered metrics handler with http.DefaultServeMux on "/debug/vars"