Feature/metrics docs

Reopening this PR, against release with addressed comments.
https://github.com/caskdata/cdap/pull/709
Example of using metrics to expose total size of words processed in bytes and the size of the longest word seen:

```java

   public static class WordsProcessor extends AbstractFlowlet {
     OutputEmitter<String> output;
     Metrics flowletMetrics; // Declare the custom metrics
     int longestWordSeen;

     @ProcessInput
     public void processInput(String name) {
       flowletMetrics.count("words.bytes", name.length);
       if (name.length > longestNameSeen) {
         longestNameSeen = name.length;
         flowletMetrics.gauge("words.longest", name.length);
       }
       output.emit(name);
     }
   }

```