HDFS-16552. Fix NPE for TestBlockManager (#4210)

(cherry picked from commit 5ebbacc48023c0c71dd428df40a4352feede373b)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/blockmanagement/TestBlockManager.java

@@ -844,6 +844,7 @@ public class TestBlockManager {
 
   @Test
   public void testSkipReconstructionWithManyBusyNodes() {
+    NameNode.initMetrics(new Configuration(), HdfsServerConstants.NamenodeRole.NAMENODE);
     long blockId = -9223372036854775776L; // real ec block id
     // RS-3-2 EC policy
     ErasureCodingPolicy ecPolicy =
@@ -889,6 +890,7 @@ public class TestBlockManager {
 
   @Test
   public void testSkipReconstructionWithManyBusyNodes2() {
+    NameNode.initMetrics(new Configuration(), HdfsServerConstants.NamenodeRole.NAMENODE);
     long blockId = -9223372036854775776L; // real ec block id
     // RS-3-2 EC policy
     ErasureCodingPolicy ecPolicy =