start () {
    return new Promise((resolve, reject) => {
      this._server = this._app.listen(this._port, () => {
        this._logger.info(`MetricsServer listening on port ${this._port}`)
        resolve()
      })
      this._app.on('error', err => reject(err))
    })
  }
Start the metrics server: Listen on a TCP port and serve metrics over HTTP