fix: service monitor to include service account token (of prom pods) as the bearer token when scraping

[yashvardhan-kukreja]

What this PR does / why we need it: re-configures the servicemonitor manifest under the helm chart to use serviceAccountToken (of the Prom pods) as the bearer token when scraping the metrics-server.

This is because metrics-server is gated by such an authz that it could be only passed by the privileges of the serviceAccount token of the Prom pods.

Else, 403 forbidden would be returned (which has been happening as of now whenever the current service monitor is applied.)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1449

Screenshot before this pull request

Original Service Monitor causing this issue:

apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: metrics-server-sm
  namespace: kube-system
  labels:
    release: kube-prometheus-stack-1711317473
spec:
  namespaceSelector:
    matchNames:
      - kube-system
  selector:
    matchLabels:
      k8s-app: metrics-server
  endpoints:
    - port: https
      scheme: https
      path: /metrics
      tlsConfig:
        insecureSkipVerify: true

Screenshot after this pull request

New Service Monitor, based on this PR, which resolved the issue:

apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: metrics-server-sm
  namespace: kube-system
  labels:
    release: kube-prometheus-stack-1711317473
spec:
  namespaceSelector:
    matchNames:
      - kube-system
  selector:
    matchLabels:
      k8s-app: metrics-server
  endpoints:
    - port: https
      bearerTokenFile: /var/run/secrets/kubernetes.io/serviceaccount/token
      scheme: https
      path: /metrics
      tlsConfig:
        caFile: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
        insecureSkipVerify: true

[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yashvardhan-kukreja Once this PR has been reviewed and has the lgtm label, please assign stevehipwell for approval. For more information see the Kubernetes Code Review Process.

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[k8s-ci-robot]

Hi @yashvardhan-kukreja. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[stevehipwell]

Conversation about this PR should be on #1449.

/hold

[yashvardhan-kukreja]

Closing in favor of https://github.com/kubernetes-sigs/metrics-server/issues/1449#issuecomment-2018808123