We shouldn't fail the grpc-server (completely) by a not implemented RPC.
Failing whole server by remote request is anti-pattern and security
risk.
Refer to https://github.com/etcd-io/etcd/runs/7034342964?check_suite_focus=true#step:5:2284
```
=== RUN TestWatchRequestProgress/1-watcher
panic: not implemented
goroutine 83024 [running]:
go.etcd.io/etcd/proxy/grpcproxy.(*watchProxyStream).recvLoop(0xc009232f00, 0x4a73e1, 0xc00e2406e0)
/home/runner/work/etcd/etcd/proxy/grpcproxy/watch.go:265 +0xbf2
go.etcd.io/etcd/proxy/grpcproxy.(*watchProxy).Watch.func1(0xc0038a3bc0, 0xc009232f00)
/home/runner/work/etcd/etcd/proxy/grpcproxy/watch.go:125 +0x70
created by go.etcd.io/etcd/proxy/grpcproxy.(*watchProxy).Watch
/home/runner/work/etcd/etcd/proxy/grpcproxy/watch.go:123 +0x73b
FAIL go.etcd.io/etcd/clientv3/integration 222.813s
FAIL
```
Signed-off-by: Benjamin Wang <wachao@vmware.com>
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.
