HDFS-16299. Fix bug for TestDataNodeVolumeMetrics#verifyDataNodeVolumeMetrics (#3616)

Reviewed-by: Inigo Goiri <inigoiri@apache.org>
Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestDataNodeVolumeMetrics.java

@@ -166,7 +166,7 @@ public class TestDataNodeVolumeMetrics {
     LOG.info("syncIoMean : " + metrics.getSyncIoMean());
     LOG.info("syncIoStdDev : " + metrics.getSyncIoStdDev());
 
-    LOG.info("readIoSampleCount : " + metrics.getReadIoMean());
+    LOG.info("readIoSampleCount : " + metrics.getReadIoSampleCount());
     LOG.info("readIoMean : " + metrics.getReadIoMean());
     LOG.info("readIoStdDev : " + metrics.getReadIoStdDev());