[DISCUSS] KIP-1250: Add metric to track size of in-memory state stores

[Evan Zhou via dev]

Hi all,

I'd like to start the discussion for KIP-1250, which adds a new metric to
track the size of in-memory state stores. Today, a similar metric exists in
Kafka, but only for RocksDB, and this KIP intends to close that gap.

https://cwiki.apache.org/confluence/x/noTMFw

Thanks,
Evan