[kube-prometheus-stack] pvc storage alert by percentage #6155
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What this PR does / why we need it
Adds a new alert that shows current storage usage percentage.
Problem: The existing KubePersistentVolumeFillingUp alert only says "storage will be full in 4 days" but doesn't tell you how full it is right now. Even after 4 days, it still says 4 days. This makes it hard to know if you need to act immediately.
Solution: This new alert fires when storage hits 85% usage. It tells you exactly how much storage is being used right now.
Special notes for your reviewer
Checklist
[prometheus-couchdb-exporter])