in ecs_logging/_structlog.py [0:0]
def __call__(self, _: Any, name: str, event_dict: Dict[str, Any]) -> str:
# Handle event -> message now so that stuff like `event.dataset` doesn't
# cause problems down the line
event_dict["message"] = str(event_dict.pop("event"))
event_dict = normalize_dict(event_dict)
event_dict.setdefault("log", {}).setdefault("level", name.lower())
event_dict = self.format_to_ecs(event_dict)
return self._json_dumps(event_dict)