[Hadoop Wiki] Update of "HADOOP-6728-MetricsV2" by LukeLu

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The "HADOOP-6728-MetricsV2" page has been changed by LukeLu.
The comment on this change is: Update doc to reflect patches committed to trunk.
http://wiki.apache.org/hadoop/HADOOP-6728-MetricsV2?action=diff&rev1=3&rev2=4

--------------------------------------------------

  ''This page keeps the design notes for [[https://issues.apache.org/jira/browse/HADOOP-6728|HADOOP-6728]] only. Current dev/user documentation for metrics system should be kept elsewhere (say, package.html and/or package-info.java in respective packages).''
+ 
+ <<TableOfContents(3)>>
  
  == Scope ==
  
@@ -128, +130 @@

  DefaultMetricsSystem.initialize("JobTracker");
  
  // Create the metrics source object
- MyMetrics myMetrics = new MyMetrics().registerWith(DefaultMetricsSystem.INSTANCE);
+ MyMetrics myMetrics = new MyMetrics().registerWith(DefaultMetricsSystem.instance());
  
  // Update the metrics
  myMetrics.foo.set(someValue);
@@ -180, +182 @@

      for (MetricTag tag : rec.tags())
        echoTag(tag.getName(), tag.getValue());
    
-     for (Metric metric : rec.metrics())
+     for (AbstractMetric metric : rec.metrics())
        metric.visit(this);
    }