Add the metrics names to the `Recorder`.

def on_train_begin(self, **kwargs):
        "Add the metrics names to the `Recorder`."
        self.names = ifnone(self.learn.loss_func.metric_names, [])
        if not self.names: warn('LossMetrics requested but no loss_func.metric_names provided')
        self.learn.recorder.add_metric_names(self.names)