[CVEDB] Fixes #4578(metric_finder function returns "unknown"

[vedpawar2254]

Fixes #4578 ([CVEDB] Why does the function metric_finder returns unknown or a metrics_id)

1. Added "UNKNOWN" Metric
2. Implemented a method (ensure_unknown_metric) to ensure it exists
3. and updated the metric_finder function

I think this should be good enough, let me know any changes are needed

[terriko]

Looks like this contains your other PR. Quick tip: when making a new branch, make sure to start from one that doesn't have any changes in it. My norm is to have my "main" branch be kept directly in sync with "main" on intel/cve-bin-tool so you can use that as the base for all PRs. We've got some instructions here on how to do that: https://github.com/intel/cve-bin-tool/blob/main/CONTRIBUTING.md#making-a-new-branch--pull-request

That said, I've approved the tests to run anyhow so you can get some results. Just beware that if anything fails in your 3.8 PR it's going to fail here too and might make it harder to see if anything in the new code is causing issues.

[vedpawar2254]

Ok thanks for the tip i'll do that also the tests failed, ill look into it and fix it