Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/2158#discussion_r68562330
--- Diff: docs/apis/common/index.md ---
@@ -1350,3 +1350,211 @@ You may specify program arguments before the job is
executed. The plan visualiza
the execution plan before executing the Flink job.
{% top %}
+
+Metrics
+-------------------
+
+Flink exposes a metric system that allows users gather and expose metrics
to external systems.
+
+### Registering metrics
+
+You can access the metric system from any user function that extends
[RichFunction]({{ site.baseurl }}/apis/common/index.html#rich-functions) by
calling getRuntimeContext().getMetricGroup().
+This method returns a MetricGroup object on which you can create and
register new metrics.
+
+If you want to count the number of records your user function has received
you could use a Counter like this:
+
+{% highlight java %}
+
+public class MyMapper extends RichMapFunction<String, Integer> {
+ private Counter counter;
+
+ @Override
+ public void open(Configuration config) {
+ // create and register a counter
+ this.counter =
getRuntimeContext().getMetricGroup().counter("myCounter");
+ ...
+ }
+
+ @public Integer map(String value) throws Exception {
+ // increment counter
+ this.counter.inc();
+ ...
+ }
+}
+
+{% endhighlight %}
+
+### Metric types
+
+Flink supports Counters, Gauges and Histograms.
+
+A Counter is used to count something. The current value can be in- or
decremented using `inc([long n])` or `dec([long n])`.
+You can create and registers a Counter by calling `counter(String name)`
on a MetricGroup. Alternatively, you can pass an instance of your own Counter
implementation along with the name.
--- End diff --
register without *s*
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