langchain/callbacks/mlflow_callback.py/MlflowCallbackHandler/on_chain_start
class MlflowCallbackHandler:    def on_chain_start(
        self, serialized: Dict[str, Any], inputs: Dict[str, Any], **kwargs: Any
    ) -> None:
        """Run when chain starts running."""
        self.metrics["step"] += 1
        self.metrics["chain_starts"] += 1
        self.metrics["starts"] += 1

        chain_starts = self.metrics["chain_starts"]

        resp: Dict[str, Any] = {}
        resp.update({"action": "on_chain_start"})
        resp.update(flatten_dict(serialized))
        resp.update(self.metrics)

        self.mlflg.metrics(self.metrics, step=self.metrics["step"])

        chain_input = ",".join([f"{k}={v}" for k, v in inputs.items()])
        input_resp = deepcopy(resp)
        input_resp["inputs"] = chain_input
        self.records["on_chain_start_records"].append(input_resp)
        self.records["action_records"].append(input_resp)
        self.mlflg.jsonf(input_resp, f"chain_start_{chain_starts}")