Attaches a user label to a client.

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))