[ONOS-3535] Accumulate control message stats using monitor service

- Augment the control message class to have device id
- Initial implementation of accumulating control message stats
- Add more adpator for unit test
- Change Collection<ControlMessage> to Set<ControlMessage>
- Fix the arithmatic exception
- Fix some javadoc warnings

Change-Id: I2abaf0d91edca5419b26f1c5a69246bcdb9201bf
diff --git a/apps/cpman/api/src/main/java/org/onosproject/cpman/MetricsDatabase.java b/apps/cpman/api/src/main/java/org/onosproject/cpman/MetricsDatabase.java
index 9d4ce08..54167b8 100644
--- a/apps/cpman/api/src/main/java/org/onosproject/cpman/MetricsDatabase.java
+++ b/apps/cpman/api/src/main/java/org/onosproject/cpman/MetricsDatabase.java
@@ -141,6 +141,7 @@
          * Add a new metric to be monitored.
          *
          * @param metricType control metric type
+         * @return builder object
          */
         Builder addMetricType(String metricType);