HADOOP-18302. Fix CheckStyle.

Author: slfan1989

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MetricsRecordImpl.java

@@ -22,11 +22,13 @@
 
 import static org.apache.hadoop.util.Preconditions.*;
 
+import org.apache.hadoop.classification.VisibleForTesting;
 import org.apache.hadoop.metrics2.MetricsInfo;
 import org.apache.hadoop.metrics2.AbstractMetric;
 import org.apache.hadoop.metrics2.MetricsTag;
 import static org.apache.hadoop.metrics2.util.Contracts.*;
 
+@VisibleForTesting
 public class MetricsRecordImpl extends AbstractMetricsRecord {
   protected static final String DEFAULT_CONTEXT = "default";