def METHOD_NAME(self, key, value):
    """
    Set a string key value pair in the request headers dictionary.
    Return the same FContext to allow for call chaining. Changing the
    op ID or correlation ID is disallowed.
    Args:
        key: string key to set in request headers
        value: string value to set for the given key
    Returns:
        FContext
    Throws:
        TypeError: if user passes non-string for key or value.
    """
    self._check_string(key)
    self._check_string(value)
    self._request_headers[key] = value
    return self