fix(helm): add Prometheus ServiceMonitor support for operator metrics - #36

Summary

Add optional Prometheus Operator ServiceMonitor support to the Helm chart so the GitHub Platform Operator metrics endpoint can be scraped without requiring users to create monitoring resources manually.

Changes

• Add optional metrics.serviceMonitor Helm configuration.
• Add a ServiceMonitor for the operator metrics service.
• Support configurable scrape interval and timeout.
• Support additional labels and annotations for Prometheus Operator discovery.
• Support an optional Prometheus ServiceAccount for accessing the secured /metrics endpoint.
• Create the required metrics-reader binding when a Prometheus ServiceAccount is configured.
• Extend values.schema.json with the new monitoring configuration.
• Document the ServiceMonitor configuration in the Helm chart README.

Example

metrics:
  enabled: true
  secure: true
  port: 8443

  serviceMonitor:
    enabled: true
    namespace: monitoring

    additionalLabels:
      release: kube-prometheus-stack

    interval: 30s
    scrapeTimeout: 10s

    prometheusServiceAccount:
      name: kube-prometheus-stack-prometheus
      namespace: monitoring