This is to aid with debugging the effectiveness of systems that
manually take care of cluster compaction, and have greater visibity
into recent compactions.
It can be handy to alert on the exactly how long it was since a
compaction (and also to put on dashboards) had happened.
---
Tested using a test cluster, the final result looks like this:
```
root@etcd-1:~# ETCDCTL_API=3 /tmp/test-etcd/etcdctl --endpoints=192.168.232.10:2379 compact 1012
compacted revision 1012
root@etcd-1:~# curl -s 192.168.232.10:2379/metrics | grep last
# HELP etcd_debugging_mvcc_db_compaction_last The unix time since the last db compaction. Resets to 0 on start.
# TYPE etcd_debugging_mvcc_db_compaction_last gauge
etcd_debugging_mvcc_db_compaction_last 1.595873939e+09
```
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
