Append new metrics to selected dataframes.

        Parameters
        ----------
        metrics : metric.EvalMetric
            New metrics to be added.
        df_name : str
            Name of the dataframe to be modified.

def append_metrics(self, metrics, df_name):
        """Append new metrics to selected dataframes.

        Parameters
        ----------
        metrics : metric.EvalMetric
            New metrics to be added.
        df_name : str
            Name of the dataframe to be modified.
        """
        dataframe = self._dataframes[df_name]
        _add_new_columns(dataframe, metrics)
        dataframe.loc[len(dataframe)] = metrics