Eu usaria a diretiva split_clients
e definiria dois upstreams.
upstream apps {
server 10.134.13.38:80; # app-00
server 10.134.13.46:80; # app-01
server 10.134.24.30:80; # app-02
}
upstream apps_new_test {
server 10.134.8.153:80; # app-new-test
}
split_clients "${remote_addr}AAA" $upstream_app {
10% apps_new_test;
* apps;
}
server {
...
proxy_pass http://$upstream_app;
}