clientv3/watch.go
func (w *watchGrpcStream) unicastResponse(wr *WatchResponse, watchId int64) bool {
	ws, ok := w.substreams[watchId]
	if !ok {
		return false
	}
	select {
	case ws.recvc <- wr:
	case <-ws.donec:
		return false
	}
	return true
}
func (w *watchGrpcStream) unicastResponse(wr *WatchResponse, watchId int64) bool {
	ws, ok := w.substreams[watchId]
	if !ok {
		return false
	}
	select {
	case ws.recvc <- wr:
	case <-ws.donec:
		return false
	}
	return true
}
// unicastResponse sends a watch response to a specific watch substream.