docstr: Append a header, preserving any duplicate entries.
code: def append(self, key: str, value: str) -> None:
      """
      Append a header, preserving any duplicate entries.
      """
  
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 7 for calling the Metadata obj's underlying member method code with example usage: obj.append(key, value) without return types