docs: def AddClientLabels(self, client_id, owner, labels):
      """Attaches a user label to a client."""
      if client_id not in self.metadatas:
        raise db.UnknownClientError(client_id)
  
      labelset = self.labels.setdefault(client_id, {}).setdefault(owner, set())
      for l in labels:
        labelset.add(utils.SmartUnicode(l))