fix: handle unknown values in metric_finder (Fixes #4578)

[vedpawar2254]

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

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

I removed the commits from the other PR, you can check this one out while i'll look into why the tests for #4654 are failing

Thanks and lemme know if we need to change anything

[terriko]

Looks like you're failing a couple of lint checks because there's some whitespace on a blank line. I've added a comment where I think it's happening, but the best way to be sure it's fixed is to run black cve_bin_tool/cvedb.py then check your file back in and push it to the PR branch.

Here's some more info on our linters in case you've never run those before:
https://github.com/intel/cve-bin-tool/blob/main/CONTRIBUTING.md#running-linters

It's also complaining about the PR title because it doesn't conform to the commit message format we use, which is https://www.conventionalcommits.org/ -- I'll change the title for you now so it should pass next time but the linter won't run again until you update the branch.

[terriko]

The blank line here has some extra spaces or a tab in it.

[vedpawar2254]

Hi @terriko , it should be good now, my vs code extensions are messing with me 😅, also there are some broken links in MANUAL.md as pointed out by @Molkree, i'll raise a pr to fix it, should i also create a issue.
I think I'll also checkout the other docs to see if there are any broken links or smtg