mvcc: Add metric for count of db key revisions compacted.

[jpbetz]

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.

[jpbetz]

@heyitsanthony Here's one of the metrics that might help us in the future.

[heyitsanthony]

would s/Revisions/Keys be better? There may be confusion between an etcd revision (mainrev) and a backend revision (mainrev + subrev).

[codecov-io]

Codecov Report

Merging #8280 into master will decrease coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #8280      +/-   ##
==========================================
- Coverage   76.42%   76.23%   -0.19%     
==========================================
  Files         346      346              
  Lines       27055    27059       +4     
==========================================
- Hits        20676    20628      -48     
- Misses       4901     4940      +39     
- Partials     1478     1491      +13

Impacted Files	Coverage Δ
mvcc/kvstore_compaction.go	79.41% <100%> (+1.99%)	⬆️
mvcc/metrics.go	96% <100%> (+0.16%)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
auth/simple_token.go	86.79% <0%> (-6.61%)	⬇️
pkg/adt/interval_tree.go	87.38% <0%> (-3.61%)	⬇️
etcdserver/api/v3rpc/watch.go	90.14% <0%> (-3.29%)	⬇️
integration/bridge.go	89% <0%> (-3%)	⬇️
clientv3/concurrency/election.go	80% <0%> (-2.41%)	⬇️
pkg/netutil/netutil.go	62.35% <0%> (-2.36%)	⬇️
... and 9 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 608df0f...2d2c076. Read the comment docs.

[jpbetz]

@heyitsanthony s/Revisions/Keys sounds good. I'll update the PR.

[heyitsanthony]

lgtm thanks

[xiang90]

lgtm