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scheduler: remove nested retry loops and fix serving metrics under healthz server port

This PR fixes the issues of #63404:

- The Serve func for metrics and healthz does not block. Inside, there is a retry loop. This PR fixes this and gets rid of the error messages every 5 seconds.
- The separate metrics server is only to be started if it is configured on another port. Before this PR we were wrongly checking for the healthz service to be activated and then launched the metrics server instead. Because both server startups are in a go routine, they were racing against each other. If you were unlucky, the metrics endpoint was winning and it returned 404 on /healthz (while it should not have been active in the first place). The kubemark tests run the scheduler with a liveness probe which failed, restarting the scheduler after some minutes.