CHANGELOG: update changelog-3.4 with learner

[jingyih]

Update CHANGELOG-3.4 from #10725.

cc @xiang90 @gyuho @jpbetz @WIZARD-CXY

[codecov-io]

Codecov Report

Merging #10767 into master will increase coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10767      +/-   ##
==========================================
+ Coverage   62.47%   63.35%   +0.87%     
==========================================
  Files         392      392              
  Lines       37158    37158              
==========================================
+ Hits        23215    23541     +326     
+ Misses      12381    12030     -351     
- Partials     1562     1587      +25

Impacted Files	Coverage Δ
proxy/httpproxy/director.go	61.42% <0%> (-18.58%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
clientv3/concurrency/election.go	79.68% <0%> (-2.35%)	⬇️
etcdserver/v3_server.go	67.88% <0%> (-1.83%)	⬇️
pkg/adt/interval_tree.go	86.18% <0%> (-1.81%)	⬇️
etcdserver/api/rafthttp/msgappv2_codec.go	69.56% <0%> (-1.74%)	⬇️
clientv3/watch.go	91.13% <0%> (-1.06%)	⬇️
embed/serve.go	36.48% <0%> (-0.91%)	⬇️
etcdserver/cluster_util.go	54.94% <0%> (-0.8%)	⬇️
... and 18 more

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[gyuho]

awesome thanks