[FLINK-3354] Determine correct size for RocksDB snapshots

Closes #1638

flink-contrib/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/AbstractRocksDBState.java

@@ -421,7 +421,8 @@ public abstract class AbstractRocksDBState<K, N, S extends State, SD extends Sta
 
 		@Override
 		public final long getStateSize() throws Exception {
-			return 0;
+			FileSystem fs = FileSystem.get(backupUri, HadoopFileSystem.getHadoopConfiguration());
+			return fs.getContentSummary(new Path(backupUri)).getLength();
 		}
 	}