def format_metrics(metrics):
    """
    Method to return dict with formatted metrics keys.
    From tuple of strings to string with dunder ('__') between values

    :param metrics: unformatted metrics. keys looks like ('test', 'AUC')
    :type metrics: dict
    :return: formatted metrics. keys looks like 'test__AUC'
    :rtype: dict
    """

    formatted_metrics = dict()
    for key, value in metrics.items():
        formatted_metrics['{}__{}'.format(key[0], key[1])] = value

    return formatted_metrics