Creates a JSON-able representation of this object.

    Returns:
      A dictionary mapping key to EventTrackerDescription (which can be used to
      create event trackers).

def create_jsonable_history(self):
    """Creates a JSON-able representation of this object.

    Returns:
      A dictionary mapping key to EventTrackerDescription (which can be used to
      create event trackers).
    """
    return {value_category_key: tracker.get_description()
            for (value_category_key, tracker) in self._trackers.items()}