_onEvent(event, transport) {
    function transportEvent(err) {
      // https://github.com/winstonjs/winston/issues/1364
      if (event === 'error' && !this.transports.includes(transport)) {
        this.add(transport);
      }
      this.emit(event, err, transport);
    }

    if (!transport['__winston' + event]) {
      transport['__winston' + event] = transportEvent.bind(this);
      transport.on(event, transport['__winston' + event]);
    }
  }
Bubbles the `event` that occured on the specified `transport` up
from this instance.
@param {string} event - The event that occured
@param {Object} transport - Transport on which the event occured
@private