def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def __getattr__(self, name: str) -> Any:
        """Custom __getattr__ to allow access to metrics schema fields."""

        if name not in self._metric_fields:
            raise AttributeError('No field: {} in metrics.'.format(name))

        return self._values[name]