docs: 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