Improve test coverage for optional metrics

[jefferbrecht]

There is an opportunity to improve the test coverage for optional metrics introduced in #456. The current behaviour is that metrics with optional: true are completely skipped during the test. It would be better if we could take a best-effort approach to validate any optional metrics that happen to show up during the test.

[github-actions]

This issue was marked stale due to lack of activity. It will be closed in 14 days.