Usage metrics is GitLab-KAS only, and does not involve agent

What does this MR do?

Fix /usage_metrics endpoint to not require agent auth. It's still protected by the kas JWT though.

Related issue: #245667 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry, as behind feature flag kubernetes_agent_internal_api
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.