HADOOP-15067. GC time percentage reported in JvmMetrics should be a gauge, not counter. Contributed by Misha Dmitriev.

(cherry picked from commit d162252d7a7223631ff66ba0210953296407e55f)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/source/JvmMetrics.java

@@ -188,7 +188,7 @@ public class JvmMetrics implements MetricsSource {
     }
 
     if (gcTimeMonitor != null) {
-      rb.addCounter(GcTimePercentage,
+      rb.addGauge(GcTimePercentage,
           gcTimeMonitor.getLatestGcData().getGcTimePercentage());
     }
   }

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/metrics2/source/TestJvmMetrics.java

@@ -101,7 +101,7 @@ public class TestJvmMetrics {
     verify(rb).tag(SessionId, "test");
     for (JvmMetricsInfo info : JvmMetricsInfo.values()) {
       if (info.name().equals("GcTimePercentage")) {
-        verify(rb).addCounter(eq(info), anyInt());
+        verify(rb).addGauge(eq(info), anyInt());
       }
     }
   }