fix(discovery): prevent duplicate GVKs and avoid stop channel leaks

[mrueg]

What this PR does / why we need it:

In CRDiscoverer.AppendToMap, GVKs were unconditionally appended to the internal map and new stop channels were created on every call. This could lead to duplicate metric registration and leaked channels/goroutines when existing stop channels were overwritten.

This change:

• Only creates a new stop channel if one does not already exist for the GVK.
• Checks for existing Kinds in the discovery map before appending to prevent duplicates.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes: (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged)
Fixes #

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[alexandernorth]

Hi @mrueg , I wanted to quickly jump in as the work I did in #2854 should also address this issue (#2870 would still be needed, as the stopCh still exist although I think it might be more appropriate to use child contexts instead of these floating channels). Happy to support or discuss my PR further, as it changes how discovery is handled.