docs: def get_handler_fp(logger):
      """
      Get handler_fp.
      This method is integrated to LoggerFactory Object in the future.
      :param logging.Logger logger: Python logging.Logger. logger instance.
      :rtype: logging.Logger.handlers.BaseRotatingHandler
      :return: Handler or Handler's stream. We call it `handler_fp`.
      """
      if not hasattr(logger, 'handlers'):
          raise blackbird.utils.error.BlackbirdError(
              'Given logger is not logging.Logger instance!'
          )
  
      if len(logger.handlers) != 1:
          raise blackbird.utils.error.BlackbirdError(
              'Given logger has invalid handlers.'
          )
  
      if hasattr(logger.handlers[0], 'stream'):
          return logger.handlers[0].stream
  
      # case of setting SysLogHandler to logger.handlers[0]
      return logger.handlers[0]