KIP-1250 — Add metric to track size of in-memory state stores

Accepted Kafka 4.3 StreamsMetrics

Adds a num-keys metric to in-memory Kafka Streams state stores (InMemoryKeyValueStore, InMemorySessionStore, etc.) tracking the current number of entries. RocksDB stores expose an estimated key count metric, but in-memory stores emit no equivalent, leaving operators without visibility into memory pressure from state store growth.

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