Store a list of metric identifiers for the given instance key and Mor
        object name.
        If the key is not in the cache, raises a KeyError.
        If the Mor object is not in the cache, raises a MorNotFoundError

def set_metrics(self, key, name, metrics):
        """
        Store a list of metric identifiers for the given instance key and Mor
        object name.
        If the key is not in the cache, raises a KeyError.
        If the Mor object is not in the cache, raises a MorNotFoundError
        """
        with self._mor_lock:
            mor = self._mor[key].get(name)
            if mor is None:
                raise MorNotFoundError("Mor object '{}' is not in the cache.".format(name))
            mor['metrics'] = metrics