Piotr Tabor 5f9a1394db integration,proxy: Skip WatchRequestProgress test in grpc-proxy mode.
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
```
2020-09-19 17:27:07 +02:00
..
2020-02-12 10:30:09 -08:00