Parser should track statistics during parsing

Our parser supports skipping events with no metadata (JMC-6846). It would be great I the parser supported exposing details around the parsing, such as how many events were parsed per event type (and their total size), and how many events that were skipped, per event id.

To do this we need to break the clean abstraction somewhat. One, perhaps not so horrid, way to do this, would be to add a new interface that implementations of IItemCollection can optionally implement. This would at least not pollute the IItemCollection interface with JFR specific stuff, but it would require an instanceof/cast where it is used.