Add CollectionUsage.getUsed() to MemoryPool gauges

[Menthalion]

For monitoring memory usage, the memory used gauge is not very suitable to put automated analysis on, because it can be varying wildly. A better choice for these situations would be the JMX metric of the memory that's still in use after the last garbage collection. This gives a more accurate indication of retained memory, without varying short lived object allocation.

This pull requests adds it as a gauge to the memory pool gauge set with postfix "used-after-gc".

The rest of the CollectionUsage methods (init, committed, max) are of no value, since they're basically identical to the normal MemoryUsage of the same pool, and are therefore not included.