_addHandler(handler) {
    if (!this.handlers.has(handler)) {
      handler.handleExceptions = true;
      const wrapper = new ExceptionStream(handler);
      this.handlers.set(handler, wrapper);
      this.logger.pipe(wrapper);
    }
  }
Helper method to add a transport as an exception handler.
@param {Transport} handler - The transport to add as an exception handler.
@returns {void}