chore: Reduce logging on index_set.go

[DylanGuedes]

What this PR does / why we need it:
We're logging number of index files on every index-set interaction. The line itself isn't useful but in case we miss it, we should probably just turn it into a metric (it is literally only showing number of index files).

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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