mvcc: add "etcd_mvcc_put_size_in_bytes" metrics

[YoyinZyc]

related #11365
@logicalhan @jingyih Please have a look.

[codecov-io]

Codecov Report

Merging #11374 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #11374      +/-   ##
==========================================
+ Coverage   64.34%   64.64%   +0.29%     
==========================================
  Files         403      403              
  Lines       37969    37973       +4     
==========================================
+ Hits        24432    24548     +116     
+ Misses      11887    11779     -108     
+ Partials     1650     1646       -4

Impacted Files	Coverage Δ
mvcc/metrics_txn.go	100% <100%> (ø)	⬆️
mvcc/metrics.go	91.04% <100%> (+0.13%)	⬆️
etcdserver/api/v3rpc/lease.go	69.31% <0%> (-5.69%)	⬇️
clientv3/maintenance.go	40.81% <0%> (-2.05%)	⬇️
clientv3/op.go	72.08% <0%> (-1.25%)	⬇️
etcdserver/cluster_util.go	56.52% <0%> (-0.8%)	⬇️
raft/raft.go	89.91% <0%> (-0.64%)	⬇️
clientv3/leasing/cache.go	87.22% <0%> (-0.56%)	⬇️
etcdserver/server.go	68.53% <0%> (-0.2%)	⬇️
pkg/transport/timeout_conn.go	80% <0%> (ø)	⬆️
... and 21 more

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[jingyih]

Looking good. Added few nits.

[jingyih]

LGTM

Might as well add for range and delete?

[logicalhan]

Ah this is great! Thanks so much!

/lgtm