docstr: If the header ``key`` does not exist, then set it to ``value``.
  Returns the header value.
code: def setdefault(self, key: str, value: str) -> str:
      """
      If the header ``key`` does not exist, then set it to ``value``.
      Returns the header value.
      """
  
imports: from __future__ import annotations
  import contextlib
  import contextvars
  import os
  import typing
  from abc import ABC
  from abc import abstractmethod
  from typing import TYPE_CHECKING
  import attr
  import starlette.datastructures
  from .utils.http import Cookie
label_desc: Use image node_id 6 for calling the Metadata obj's underlying member method code with example usage: obj.setdefault(key, value) without return types