applyUserInfoMiddleware.go

var forwardUserID = gateway.RequestMiddleware(func(r *http.Request) error {
    // the initial context of the request is set to match the one we modified earlier

    // we are safe to extract the value we saved in context
    if userID := r.Context().Value("user-id"); userID != nil {
        // set the header with the value we pulled out of context
        r.Header.Set("USER_ID", userID.(string))
    }

    // there was no error
    return nil
})