HADOOP-14892. MetricsSystemImpl should use Time.monotonicNow for measuring durations. Contributed by Chetna Chaudhari.

(cherry picked from commit 3a10367a17b99e2c3035ec54048f94f334d93227)

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

@@ -415,10 +415,10 @@ public class MetricsSystemImpl extends MetricsSystem implements MetricsSource {
 
   private void snapshotMetrics(MetricsSourceAdapter sa,
                                MetricsBufferBuilder bufferBuilder) {
-    long startTime = Time.now();
+    long startTime = Time.monotonicNow();
     bufferBuilder.add(sa.name(), sa.getMetrics(collector, true));
     collector.clear();
-    snapshotStat.add(Time.now() - startTime);
+    snapshotStat.add(Time.monotonicNow() - startTime);
     LOG.debug("Snapshotted source "+ sa.name());
   }
 
@@ -431,7 +431,7 @@ public class MetricsSystemImpl extends MetricsSystem implements MetricsSource {
   synchronized void publishMetrics(MetricsBuffer buffer, boolean immediate) {
     int dropped = 0;
     for (MetricsSinkAdapter sa : sinks.values()) {
-      long startTime = Time.now();
+      long startTime = Time.monotonicNow();
       boolean result;
       if (immediate) {
         result = sa.putMetricsImmediate(buffer); 
@@ -439,7 +439,7 @@ public class MetricsSystemImpl extends MetricsSystem implements MetricsSource {
         result = sa.putMetrics(buffer, logicalTime);
       }
       dropped += result ? 0 : 1;
-      publishStat.add(Time.now() - startTime);
+      publishStat.add(Time.monotonicNow() - startTime);
     }
     droppedPubAll.incr(dropped);
   }