docs: def FieldDefinitionTuplesFromProtos(field_def_protos):
    """Converts MetricFieldDefinition protos to (field-name, type) tuples."""
    # TODO: This needs fixing for Python 3.
    field_def_tuples = []
    for proto in field_def_protos:
      if proto.field_type == rdf_stats.MetricFieldDefinition.FieldType.INT:
        field_type = int
      elif proto.field_type == rdf_stats.MetricFieldDefinition.FieldType.STR:
        # Use old style str in Python 2 here or the streamz library will break.
        field_type = compatibility.builtins.str
      else:
        raise ValueError("Unknown field type: %s" % proto.field_type)
      field_def_tuples.append((proto.field_name, field_type))
    return field_def_tuples