def AddLabel(self, label):
    """Adds a label to the event tag.

    Args:
      label (str): label.

    Raises:
      TypeError: if the label provided is not a string.
      ValueError: if a label is malformed.
    """
    if not isinstance(label, py2to3.STRING_TYPES):
      raise TypeError('label is not a string type. Is {0:s}'.format(
          type(label)))
    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))

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

    Args:
      label (str): label.

    Raises:
      TypeError: if the label provided is not a string.
      ValueError: if a label is malformed.