mvcc: set db size metric on restore

[heyitsanthony]

Fixes #8080

[xiang90]

lgtm

[xiang90]

even if there is nothing changed, we still close/reopen a txn every 100ms, right? i thought the db size should get updated at that time.

[heyitsanthony]

@xiang90 it's an mvcc metric so the backend doesn't have any control over it; the mvcc code only sets the metric after processing a write

[heyitsanthony]

failing on grpc/grpc-go#1316 and #5847. Jumping the gate and merging since this is low risk.