code: def AddEventAttribute(self, attribute_name, attribute_value):
      
      if attribute_name in self._extra_event_attributes:
        raise KeyError('Event attribute {0:s} already set'.format(
            attribute_name))
  
      self._extra_event_attributes[attribute_name] = attribute_value
docs: Adds an attribute that will be set on all events produced.
  
      Setting attributes using this method will cause events produced via this
      mediator to have an attribute with the provided name set with the
      provided value.
  
      Args:
        attribute_name (str): name of the attribute to add.
        attribute_value (str): value of the attribute to add.
  
      Raises:
        KeyError: if the event attribute is already set.