[3.4] Backport #12671 clientv3: Replace balancer with upstream grpc solution

[chaochn47]

Breakdown of #16827

Backports

• clientv3: Replace balancer with upstream grpc solution #12671
• clientv3: PS: Replace balancer with upstream grpc solution #12706

Changes made on top of the original PR:

• Unrelated go.mod and go.sum files are removed
• Internal resolver and endpoints packages are moved into clientv3 path
• Stripped grpc proxy changes

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[chaochn47]

Proxy test failed. https://github.com/etcd-io/etcd/actions/runs/6672706002/job/18137135993?pr=16844

I have to backport its follow up PR to make CI green.

[ahrtr]

Stripped grpc proxy changes

There are two changes with grpc proxy in #12706,

• proxy/grpcproxy/health.go
  It should be fine to ignore this change, because the c will never be nil.
• etcdmain/grpc_proxy.go
  Why stripped this change?

[chaochn47]

Stripped grpc proxy changes

There are two changes with grpc proxy in #12706,

* proxy/grpcproxy/health.go
  It should be fine to ignore this change, because the `c` will never be nil.

* etcdmain/grpc_proxy.go
  Why stripped this change?

etcdmain/grpc_proxy.go change is unrelated to unblock the CVE change. The client was created for proxy health check in #12114.

I assume we want to reduce the moving parts for the sake of bumping up gRPC versions, so I stripped this change.

Does that make sense to you? @ahrtr

[ahrtr]

The client was created for proxy health check in #12114.

OK, This PR (a feature) hasn't been backported to 3.4 at all, so no need to backport the grpc-proxy related change in #12706 to 3.4 either.

[chaochn47]

The client was created for proxy health check in #12114.

OK, This PR (a feature) hasn't been backported to 3.4 at all, so no need to backport the grpc-proxy related change in #12706 to 3.4 either.

Yeah, I could have documented it clearer.

@ahrtr Other than this contentious change, is there anything else I should follow up for this PR?

[ahrtr]

@ahrtr Other than this contentious change, is there anything else I should follow up for this PR?

Overall looks good to me. The only minor comment is #16844 (comment)

Thanks.

[ahrtr]

Just merged #16849, please rebase this PR.

[ahrtr]

LGTM

Thanks @chaochn47

cc @jmhbnz @serathius @wenjiaswe @tjungblu

[jmhbnz]

Backports LGTM - Thanks @chaochn47