fix: Use rest.HTTPClientFor to create TLS-configured client for metrics authentication

[jianzhangbjz]

Description of the change:

Problem

Prometheus fails to scrape OLM metrics with HTTP 500 errors. Analysis shows the root cause is:

  E1024 08:45:54 Failed to make webhook authenticator request:
  Post "https://172.30.0.1:443/apis/authentication.k8s.io/v1/tokenreviews":
  tls: failed to verify certificate: x509: certificate signed by unknown authority

The authentication filter uses TokenReview API to validate bearer tokens from Prometheus, but the API call fails due to TLS certificate verification errors.

Motivation for the change:

Architectural changes:

• Extracts CA certificates from config.CAData or config.CAFile
• Creates a TLS transport with proper root CA pool
• Returns a fully configured client that can verify API server certificates

Testing remarks:

• Added comprehensive unit tests (10 test cases, all passing)
• Tests verify TLS configuration is properly applied
• Tests demonstrate the bug with empty http.Client
• Tests validate CA certificate loading

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Bug fixes are accompanied by regression test(s)
• e2e tests and flake fixes are accompanied evidence of flake testing, e.g. executing the test 100(0) times
• tech debt/todo is accompanied by issue link(s) in comments in the surrounding code
• Tests are comprehensible, e.g. Ginkgo DSL is being used appropriately
• Docs updated or added to /doc
• Commit messages sensible and descriptive
• Tests marked as [FLAKE] are truly flaky and have an issue
• Code is properly formatted

Assisted-by: Claude Code

[jianzhangbjz]

Hi @tmshort , could you help take a look? Thanks!

[anik120]

Looks like the e2e tests are passing here, I've created a PR to run the metrics e2e test with these changes #3687

[anik120]

/lgtm

metrics e2e tests are passing in #3687 with these changes

[openshift-ci]

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