add Prometheus metrics

[blockspacer]

https://github.com/blockspacer/gd_fps_server/issues/256

https://github.com/blockspacer/webrtc-datachannels/blob/develop/examples/authoritativeserver/src/metrics.hh

https://github.com/blockspacer/webrtc-datachannels/blob/develop/examples/authoritativeserver/src/primitive_stats_macros.h

[blockspacer]

https://github.com/smfrpc/smf/blob/master/src/core/rpc_server.cc#L83

    // start on background co-routine
    (void)seastar::prometheus::add_prometheus_routes(*admin_, conf)
      .then([http_port = args_.http_port, admin = admin_, ip = args_.ip]() {
        return admin
          ->listen(seastar::make_ipv4_address(
            ip.empty() ? seastar::ipv4_addr{http_port}
                       : seastar::ipv4_addr{ip, http_port}))
          .handle_exception([](auto ep) {
            LOG_ERROR("Exception on HTTP Admin: {}", ep);
            return seastar::make_exception_future<>(ep);
          });
      });
  }