aggregate new_stat to origin_stat.

    Args:
        origin_stat (dict): origin stat dict, will be updated with new_stat dict.
        new_stat (dict): new stat dict.

def aggregate_stat(origin_stat, new_stat):
    """ aggregate new_stat to origin_stat.

    Args:
        origin_stat (dict): origin stat dict, will be updated with new_stat dict.
        new_stat (dict): new stat dict.

    """
    for key in new_stat:
        if key not in origin_stat:
            origin_stat[key] = new_stat[key]
        elif key == "start_at":
            # start datetime
            origin_stat[key] = min(origin_stat[key], new_stat[key])
        else:
            origin_stat[key] += new_stat[key]