def get_union_metrics(metric_a, metric_b):
    """Union of metric_a and metric_b

    :param metric_a: list
    :param metric_b: list
    :return: Union metrics list from metric_a and metric_b
    """
    if metric_a is None and metric_b is None:
        return None
    elif metric_a is None:
        return metric_b
    elif metric_b is None:
        return metric_a
    else:
        metric_list = list(set(metric_a).union(metric_b))
        return metric_list