def AddLabels(self, labels):
    """Adds labels to the event tag.

    Args:
      labels (list[str]): labels.

    Raises:
      ValueError: if a label is malformed.
    """
    for label in labels:
      if not self._VALID_LABEL_REGEX.match(label):
        raise ValueError((
            'Unsupported label: "{0:s}". A label must only consist of '
            'alphanumeric characters or underscores.').format(label))

    for label in labels:
      if label not in self.labels:
        self.labels.append(label)
Adds labels to the event tag.

    Args:
      labels (list[str]): labels.

    Raises:
      ValueError: if a label is malformed.