Add GarbageCollectorMXBean to micro-jmx-metrics #421

In order to monitor gc metrics programmatically, the following java.lang type should be added to the micro-jmx-metrics plugin
https://docs.oracle.com/javase/7/docs/api/java/lang/management/GarbageCollectorMXBean.html
The intent is to plot two metrics:

• CollectionCount: Number of GC collections triggered since JVM start
• CollectionTime: Accumulated collection time spent in seconds since JVM start