[kube-prometheus-stack] add kubeScheduler serviceMonitor additionalPath

[firasmosbehi]

Summary

This PR adds support for an additional kube-scheduler ServiceMonitor path in kube-prometheus-stack, addressing #6334.

Problem

The kube-scheduler ServiceMonitor currently renders a single endpoint scrape. Users who need metrics from additional scheduler paths (for example /metrics/resources) cannot configure this without patching manifests externally.

Root cause

templates/exporters/kube-scheduler/servicemonitor.yaml only emits one endpoint and no value exists to configure a second path.

Fix

• Added kubeScheduler.serviceMonitor.additionalPath (default "") to values.
• Updated kube-scheduler ServiceMonitor template to render a second endpoint when additionalPath is set.
• Reused the same scrape settings (port, interval, auth, TLS, relabelings) for the extra endpoint and set only path differently.
• Bumped chart version from 82.9.0 to 82.9.1.

Validation

• helm lint charts/kube-prometheus-stack
• helm template test charts/kube-prometheus-stack --set kubeScheduler.serviceMonitor.additionalPath=/metrics/resources
• Verified rendered kube-scheduler ServiceMonitor includes an additional endpoint with path: /metrics/resources.

Compatibility

No behavior change by default since additionalPath is empty unless explicitly configured.

[jkroepke]

Please make it more explcit to scrape metrics/resources as additional feature, disabled by default.