[Cherry-pick release-1.12] Fix scheduler panic when metrics are disabled

[Copilot]

Cherry-pick of #4731 to release-1.12. Scheduler panics with nil pointer dereference when started without --enable-metrics=true because Kubernetes scheduler framework plugins access k8smetrics.Goroutines unconditionally.

Changes:

• Move metrics.InitKubeSchedulerRelatedMetrics() outside the metrics-enabled conditional in cmd/scheduler/app/server.go
• Ensures metric registry initialization occurs regardless of --enable-metrics flag

// Before: initialization only when metrics enabled
if opt.EnableMetrics || opt.EnablePprof {
    metrics.InitKubeSchedulerRelatedMetrics()
    go startMetricsServer(opt)
}

// After: always initialize registry, conditionally start server
metrics.InitKubeSchedulerRelatedMetrics()

if opt.EnableMetrics || opt.EnablePprof {
    go startMetricsServer(opt)
}

The panic occurs in k8s.io/kubernetes/pkg/scheduler/framework/parallelize.Parallelizer.Until when it calls Goroutines.WithLabelValues() on a nil metric.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[release-1.12] Fix scheduler panic when metrics are disabled</issue_title>
<issue_description>Manual cherrypick required.

#4731 failed to apply on top of branch "release-1.12":

Patch is empty.
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [release-1.12] Fix scheduler panic when metrics are disabled #4837

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[volcano-sh-bot]

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[Copilot]

Pull request overview

This PR fixes a critical scheduler panic that occurs when the scheduler is started without the --enable-metrics=true flag. The panic was caused by a nil pointer dereference in k8smetrics.Goroutines, which is accessed by Kubernetes scheduler framework plugins regardless of the metrics flag setting.

Key changes:

• Move metric registry initialization outside the conditional block to ensure it always occurs
• Add explanatory comment documenting why this initialization is required unconditionally

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[JesseStutler]

/retest