Backfilling the new limits when updating the metrics

[alanprot]

What this PR does:
Follow up of #5950

This PR is adding some missing things:

• Changelog
• Add the per_labelset_series_limit dimention on the cortex_discarded_samples_total metric
• Backfill the new limits when Updating the cortex_ingester_active_series_per_labelset
  • Before this change, a new metric "matching" the new limits needed to be added so we could backfill it.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• [NA] Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]