grpc: watch metadata changes by directory

other/java/client/src/main/proto/filer.proto

@@ -342,6 +342,7 @@ message SubscribeMetadataRequest {
     int32 client_id = 7;
     int64 until_ns = 8;
     int32 client_epoch = 9;
+    repeated string directories = 10;  // exact directory to watch
 }
 message SubscribeMetadataResponse {
     string directory = 1;