*: disable v2 API by default

[gyuho]

Still need changelog.

/cc @xiang90 @jpbetz @philips

[philips]

The motivation for this is to disable it by default in v3.4?

[gyuho]

@philips Yes. We will still keep v2 storage and v2 API, until we have v4.

[philips]

OK, this will need major documentation, changelog, and user outreach. Has any of that been planned on an issue?

[gyuho]

Yeah I will make sure we highlight this in our documentation as a breaking change.

[codecov-io]

Codecov Report

Merging #10935 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10935      +/-   ##
==========================================
- Coverage   63.93%    63.8%   -0.13%     
==========================================
  Files         400      400              
  Lines       37415    37415              
==========================================
- Hits        23920    23874      -46     
- Misses      11866    11935      +69     
+ Partials     1629     1606      -23

Impacted Files	Coverage Δ
etcdserver/server.go	68.48% <ø> (+0.26%)	⬆️
embed/config.go	50.7% <ø> (ø)	⬆️
auth/store.go	45.08% <0%> (-17.76%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
clientv3/retry_interceptor.go	65% <0%> (-4.5%)	⬇️
raft/tracker/inflights.go	91.83% <0%> (-4.09%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
clientv3/concurrency/election.go	79.68% <0%> (-2.35%)	⬇️
clientv3/balancer/balancer.go	86.36% <0%> (-2.28%)	⬇️
etcdserver/api/v2http/client.go	84.3% <0%> (-1.21%)	⬇️
... and 25 more

---

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[jingyih]

FYI. Noticed some broken unit test:

https://travis-ci.com/etcd-io/etcd/jobs/219715780#L585

[gyuho]

I checked the code base, and confirmed that publish method won't be affected by this. It goes directly to rafthttp. So even with v2 client handler disabled (via --enable-v2=false), cluster can still process publish requests (which encodes requests using v2 storage). Added a TODO to replace this later with v3 storage.

[jingyih]

Agreed.

[gyuho]

@jingyih Now CI should be passing. PTAL. Thx.

[jingyih]

LGTM

[jingyih]

@gyuho I did not see a corresponding changelog for this?

[gyuho]

@jingyih Ah, totally missed that. Let me add now. Thanks.