Move server metrics to health port

[aldudko]

Similarly to the agent, run /metrics endpoint on the health port instead of the admin port.

By default, adminServer binds to localhost, which doesn't let prometheus to scrap server's /metrics.
If I bind adminServer to podIP, /debug/ endpoints will be exposed too since they are served by the same adminServer.
It is more secure to keep /debug on localhost and expose metrics on podIP:healthPort

https://kubernetes.slack.com/archives/CPS4Z8G8N/p1749655050637769

How was this tested?

./bin/proxy-server --server-ca-cert=certs/frontend/issued/ca.crt --server-cert=certs/frontend/issued/proxy-frontend.crt --server-key=certs/frontend/private/proxy-frontend.key --cluster-ca-cert=certs/agent/issued/ca.crt --cluster-cert=certs/agent/issued/proxy-frontend.crt --cluster-key=certs/agent/private/proxy-frontend.key --health-port=8134 --admin-port=8133

curl localhost:8133/metrics
<a href="//localhost:8134/metrics">Moved Permanently</a>.

curl localhost:8134/metrics
# HELP go_gc_duration_seconds A summary of the wall-time pause (stop-the-world) duration in garbage collection cycles.
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{quantile="0"} 0.000157916
go_gc_duration_seconds{quantile="0.25"} 0.000157916
...

[linux-foundation-easycla]

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• ✅ login: aldudko / name: Alex Dudko (02f0829)

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[cheftako]

/ok-to-test

[cheftako]

@ipochi I'm fine with the change. I think the only real concern would be backward compatibility. Arguably with admin port bound to localhost then it seems unlikely that anyone if relying on scraping from the port. Are you ok as is or do you think which should ask for a backward compatibility mode flag?

[jkh52]

My 2 cents: just leave the metrics handler in both places unconditionally, and add a comment that admin server is only for backward compatibility.

Otherwise, nice to add some unit test coverage of this redirect.

[cheftako]

Hi @aldudko , I think @jkh52 has a point. Can we just leave the metrics response in place on the admin server? If you can reach it, you don't need to redirect. If you can only reach the health port, then the redirect doesn't help.