Create an empty instance if it doesn't exist.
        If the instance already exists, this is a noop.

def init_instance(self, key):
        """
        Create an empty instance if it doesn't exist.
        If the instance already exists, this is a noop.
        """
        with self._lock:
            if key not in self._metadata:
                self._metadata[key] = {}
                self._metric_ids[key] = []