Copy and deliver a request to another nginx server for real traffic testing

I found the solution myself. So I share my finding here.

I used post_action directive for this.

location / {
    uwsgi_pass      unix:app.sock;
    post_action @post_action; 
}

location @post_action {
    proxy_pass      http://dst_host:dst_port; 
}

The request first delivered to unix:app.sock and when it completed, post_action directive pass the request to named location, @post_action.

It works like a charm!