langchain/callbacks/mlflow_callback.py/MlflowLogger/metric
class MlflowLogger:    def metric(self, key: str, value: float) -> None:
        """To log metric to mlflow server."""
        with self.mlflow.start_run(
            run_id=self.run.info.run_id, experiment_id=self.mlf_expid
        ):
            self.mlflow.log_metric(key, value)