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

[Evan Zhou via dev]

Hi all,

I'd like to start a vote for KIP-1250, which adds a new metric for the size
of in-memory state stores.

KIP: https://cwiki.apache.org/confluence/x/noTMFw
Discussion: https://lists.apache.org/thread/cdcyy62tgq5phq9dyso2nzd352020bp8

Thanks,
Evan