docstr: Set the header ``key`` to ``value``, removing any duplicate entries.
  Retains insertion order.
code: def __setitem__(self, key: str, value: str) -> None:
      """
      Set the header ``key`` to ``value``, removing any duplicate entries.
      Retains insertion order.
      """
  
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 1 for calling the Metadata obj's underlying member method code with example usage: obj.__setitem__(key, value) without return types