Prepare file with metric names.

def on_train_begin(self, **kwargs: Any) -> None:
        "Prepare file with metric names."
        self.path.parent.mkdir(parents=True, exist_ok=True)      
        self.file = self.path.open('a') if self.append else self.path.open('w')
        self.file.write(','.join(self.learn.recorder.names[:(None if self.add_time else -1)]) + '\n')