Fixes:
go test -tags cluster_proxy ./clientv3/integration -v -run TestWatchRequestProgress
Does not fail the grpc-server (completely) by a not implemented RPC.
Failing whole server by remote request is anti-pattern and security
risk.
Prior to the fix, the command line above was failing with:
```
=== RUN TestWatchRequestProgress/0-watcher
panic: not implemented
goroutine 602 [running]:
go.etcd.io/etcd/v3/proxy/grpcproxy.(*watchProxyStream).recvLoop(0xc0004779d0, 0x0, 0x0)
/home/ptab/corp/etcd/proxy/grpcproxy/watch.go:275 +0xac5
go.etcd.io/etcd/v3/proxy/grpcproxy.(*watchProxy).Watch.func1(0xc0034f94a0, 0xc0004779d0)
/home/ptab/corp/etcd/proxy/grpcproxy/watch.go:129 +0x53
created by go.etcd.io/etcd/v3/proxy/grpcproxy.(*watchProxy).Watch
/home/ptab/corp/etcd/proxy/grpcproxy/watch.go:127 +0x3c8
FAIL go.etcd.io/etcd/v3/clientv3/integration 0.215s
FAIL
```
grpc proxy opens additional 2 watching channels. The metric is shared
between etcd-server & grpc_proxy, so all assertions on number of open
watch channels need to take in consideration the additional "2"
channels.
This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
