Skip WatchRequestProgress test in grpc-proxy mode.

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>

clientv3/integration/watch_test.go

@@ -607,6 +607,9 @@ func TestConfigurableWatchProgressNotifyInterval(t *testing.T) {
 }
 
 func TestWatchRequestProgress(t *testing.T) {
+	if integration.ThroughProxy {
+		t.Skip("grpc-proxy does not support WatchProgress yet")
+	}
 	testCases := []struct {
 		name     string
 		watchers []string