docs: def Stat(self, urns):
      """Returns metadata about all urns.
  
      Currently the metadata include type, and last update time.
  
      Args:
        urns: The urns of the objects to open.
  
      Yields:
        A dict of metadata.
  
      Raises:
        ValueError: A string was passed instead of an iterable.
      """
      if isinstance(urns, string_types):
        raise ValueError("Expected an iterable, not string.")
      for subject, values in data_store.DB.MultiResolvePrefix(
          urns, ["aff4:type", "metadata:last"]):
        res = dict(urn=rdfvalue.RDFURN(subject))
        for v in values:
          if v[0] == "aff4:type":
            res["type"] = v
          elif v[0] == "metadata:last":
            res["last"] = rdfvalue.RDFDatetime(v[1])
        yield res