code: func Heartbeat(endpoint string) func(http.Handler) http.Handler {
  	f := func(h http.Handler) http.Handler {
  		fn := func(w http.ResponseWriter, r *http.Request) {
  			if r.Method == "GET" && strings.EqualFold(r.URL.Path, endpoint) {
  				w.Header().Set("Content-Type", "text/plain")
  				w.WriteHeader(http.StatusOK)
  				w.Write([]byte("."))
  				return
  			}
  			h.ServeHTTP(w, r)
  		}
  		return http.HandlerFunc(fn)
  	}
  	return f
  }
docs: // Heartbeat endpoint middleware useful to setting up a path like
  // `/ping` that load balancers or uptime testing external services
  // can make a request before hitting any routes. It's also convenient
  // to place this above ACL middlewares as well.