Convert the string file_version in event.proto into a float.

  Args:
    file_version: String file_version from event.proto

  Returns:
    Version number as a float.

def _ParseFileVersion(file_version):
  """Convert the string file_version in event.proto into a float.

  Args:
    file_version: String file_version from event.proto

  Returns:
    Version number as a float.
  """
  tokens = file_version.split('brain.Event:')
  try:
    return float(tokens[-1])
  except ValueError:
    ## This should never happen according to the definition of file_version
    ## specified in event.proto.
    logger.warn(
        ('Invalid event.proto file_version. Defaulting to use of '
         'out-of-order event.step logic for purging expired events.'))
    return -1