Allow for ignoring metrics reported by the missing metrics metric

[Sebastiaan127001]

Is your feature request related to a problem? Please describe.
I'm always frustrated when the 'Missing Metrics' metric reports a metric as missing in one subject while it is present in another subject. Or when the 'Missing Metrics' metric reports a metric as missing that I consider optional.

Describe the solution you'd like
Make it possible to ignore metrics that are being used in another subject or that are optional.

Add two source configuration parameters:

• Metrics types to ignore if not present in a report
• Metrics types to ignore if present at least once in a report

Update the "Add metric" dropdown menu:

• Change the "Hide metric types already used" to a dropdown with three options:
  • "Hide metric types already used in this subject"
  • "Hide metric types already used in this report"
  • "Don't hide already used metric types" (default option)