class_docstr: Raise when ``get_map_lengths`` cannot populate all mapping metadata.
  
  This is generally due to not all upstream tasks have finished when the
  function is called.
code: def __str__(self) -> str:
      keys = ", ".join(repr(k) for k in sorted(self.missing))
      return f"Failed to populate all mapping metadata; missing: {keys}"
  
imports: from __future__ import annotations
  import collections.abc
  import functools
  import operator
  from typing import TYPE_CHECKING, Any, Dict, Iterable, Mapping, NamedTuple, Sequence, Sized, Union
  import attr
  from airflow.utils.mixins import ResolveMixin
  from airflow.utils.session import NEW_SESSION, provide_session
label_desc: Use image node_id 2 for calling the NotFullyPopulated obj's underlying member method code with example usage: obj.__str__() and returns: str, keys