Append request id which got from response
    header to resource.req_ids list.

def append_request_id(req, resp, resource, params):
    """Append request id which got from response
    header to resource.req_ids list.
    """
    def get_headers(resp):
        if hasattr(resp, 'headers'):
            return resp.headers
        if hasattr(resp, '_headers'):
            return resp._headers
        return None

    if(isinstance(resp, Response) or
       (get_headers(resp) is not None)):
        # Extract 'x-request-id' from headers if
        # response is a Response object.
        request_id = get_headers(resp).get('x-request-id')
    else:
        # If resp is of type string or None.
        request_id = resp

    if resource.req_ids is None:
        resource.req_ids = []

    if request_id not in resource.req_ids:
        resource.req_ids.append(request_id)