code: def _get_environ(environ):
      # type: (Dict[str, str]) -> Iterator[Tuple[str, str]]
      
      keys = ["SERVER_NAME", "SERVER_PORT"]
      if _should_send_default_pii():
          # Add all three headers here to make debugging of proxy setup easier.
          keys += ["REMOTE_ADDR", "HTTP_X_FORWARDED_FOR", "HTTP_X_REAL_IP"]
  
      for key in keys:
          if key in environ:
              yield key, environ[key]
docs: Returns our whitelisted environment variables.