Stricter validation of prometheus label values

What does this MR do?

Downstream MR for https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/38 and https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/39

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
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Performance and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers