HADOOP-11958. MetricsSystemImpl fails to show backtrace when an error occurs (Jason Lowe via jeagles)

(cherry picked from commit 2236b577a34b069c0d1f91da99f63a199f260ac2)

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

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -161,6 +161,8 @@ Release 2.8.0 - UNRELEASED
     HADOOP-7139. Allow appending to existing SequenceFiles
     (kanaka kumar avvaru via vinayakumarb)
 
+    HADOOP-11958. MetricsSystemImpl fails to show backtrace when an error
+    occurs (Jason Lowe via jeagles)
   OPTIMIZATIONS
 
     HADOOP-11785. Reduce the number of listStatus operation in distcp

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

@@ -368,9 +368,8 @@ public class MetricsSystemImpl extends MetricsSystem implements MetricsSource {
           public void run() {
             try {
               onTimerEvent();
-            }
-            catch (Exception e) {
-              LOG.warn(e);
+            } catch (Exception e) {
+              LOG.warn("Error invoking metrics timer", e);
             }
           }
         }, millis, millis);