HBASE-15959 Fix flaky test TestRegionServerMetrics.testMobMetrics (Huaxiang Sun and Jingcheng Du)

hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionServerMetrics.java

@@ -451,6 +451,10 @@ public class TestRegionServerMetrics {
         admin.flush(tableName);
       }
       region.getTableDesc().getFamily(cf).setMobThreshold(0);
+
+      // closing the region forces the compaction.discharger to archive the compacted hfiles
+      ((HRegion) region).close();
+
       // metrics are reset by the region initialization
       ((HRegion) region).initialize();
       region.compact(true);