When digging into etcd/boltdb "storage space exceeded" issues, this metric may help answer questions about if/when compactions occured and how much data was freed.
Relying on mvcc to set the db size metric can cause it to
miss size changes when a txn commits after the last write
completes before a quiescent period. Instead, load the
db size on demand.
Fixes#8146
Gyuho Lee