langchain/callbacks/tracers/base.py/BaseTracer/_start_trace
class BaseTracer:    def _start_trace(self, run: Run) -> None:
        """Start a trace for a run."""
        if run.parent_run_id:
            parent_run = self.run_map[str(run.parent_run_id)]
            if parent_run:
                self._add_child_run(parent_run, run)
            else:
                raise TracerException(
                    f"Parent run with UUID {run.parent_run_id} not found."
                )
        self.run_map[str(run.id)] = run