How to get forwardAuth's response's metrics?

In Grafana, I tried useing query like this:

sum(delta(traefik_service_requests_total{exported_service=~"myauthsvc@kubernetes", code=~"^5.*"}[5m])) by (code)

But it shows nothing even my client was getting http status code 500.
I guess it's because forwardAuth isn't sending requests via ingress?

Is there any other way to get response metric from forwardAuth? Or I can only export my own metric from my auth service(myauthsvc)?