Routes = [
          {'_', [
                 {"/metrics/[:registry]", prometheus_cowboy1_handler, []},
                 {"/", toppage_handler, []}
                ]}
         ]

Routes = [
          {'_', [
                 {"/metrics/[:registry]", prometheus_cowboy2_handler, []},
                 {"/", toppage_handler, []}
                ]}
         ]

  {ok, _} = cowboy:start_clear(http, [{port, 0}],
                               #{env => #{dispatch => Dispatch},
                                 metrics_callback => fun prometheus_cowboy2_instrumenter:observe/1,
                                 stream_handlers => [cowboy_metrics_h, cowboy_stream_h]})