See and document which of Hibernate Core own statistics make sense
Make sure that our persisters and loaders properly populate these statistics (if their JDBC version do it)
If needed add Hibernate OGM specific statistics (new class, subclassing Hibernate Core one?)
Unit test them
Hardy, do you want to work on that? It looks like a nice one to dive safely into Hibernate OGM.
Percentile libraries: Metrix or hdrhistogram