nginx rewrite with proxy_pass
You do not need to use rewrite for your case, since you want to append _plugin/kibana to any routes.
Assuming the nginx config is for http://origin.com/:
server {
    listen 80;
    server_name origin.com;
    location / {
        proxy_set_header X-Forwared-Proto $scheme;

        # http://origin.com/req/1/2 will go to http://target.com/_plugin/kibana/req/1/2
        proxy_pass http://target.com/_plugin/kibana;

    }
}