async metrics() {
    return {
      api: await global.kuzzle.ask("kuzzle:api:funnel:metrics"),
      network: await global.kuzzle.ask("core:network:router:metrics"),
      realtime: await global.kuzzle.ask("core:realtime:hotelClerk:metrics"),
    };
  }
Fetches and returns Kuzzle core metrics
@returns {Promise<Object>}