keras/src/metrics/reduction_metrics.py/MeanMetricWrapper/get_config
class MeanMetricWrapper:    def get_config(self):
        base_config = super().get_config()
        config = {"fn": serialization_lib.serialize_keras_object(self._fn)}
        config.update(serialization_lib.serialize_keras_object(self._fn_kwargs))
        return {**base_config, **config}