25.8 stable backport of #113919: Fix KafkaConsumersWithAssignment drifting negative

[ilejn]

The KafkaConsumersWithAssignment metric is decremented twice per rebalance revocation but incremented once per assignment, so it drifts down by one per rebalance and eventually reads negative.

The revocation callback in KafkaConsumer::createConsumer decrements the gauge directly, then calls cleanAssignment, which decrements it again. The assignment callback increments it once.

Measured on a 26.7.2.59 server up 39 hours with 8 Kafka tables:

Metric	Value
KafkaConsumersWithAssignment	-60
KafkaRebalanceAssignments	76
KafkaRebalanceRevocations	68

76 - 2 * 68 = -60. Per-table num_rebalance_assignments and num_rebalance_revocations in system.kafka_consumers sum to the same 76 and 68. KafkaAssignedPartitions is decremented only in cleanAssignment and read a correct 21 on the same server.

cleanAssignment is reached by every path that drops an assignment - the revocation callback, moveConsumer and the destructor - so removing the callback's own decrement leaves exactly one decrement per assignment dropped.

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Fix the KafkaConsumersWithAssignment metric drifting negative - it was decremented twice for every consumer group rebalance.
(ClickHouse#113919 by @kalavt)

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