Primary has only the Primary-type zones it owns (10). Replicas have those
+ built-in zones (localhost, in-addr.arpa reverse, etc.), so their count
(14) can never match primary. Alert expr compared max-min across all
instances, making it chronically firing.
Fix: instance!="primary" filter. The real signal this alert wants is
"did one replica drift from the others" — replica-to-replica comparison
captures that; primary was never comparable.