Fix checkstyle

Author: LeonGao91

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/BalancerMetrics.java

@@ -27,7 +27,7 @@
  * Metrics for individual Balancer.
  */
 @Metrics(about="Balancer metrics", context="dfs")
-class BalancerMetrics {
+final class BalancerMetrics {
 
   private final Balancer balancer;