When working on JDK-8357818, Ramki found `increment_total_collections` isn't used in shenandoah gc [1], which leads to inaccurate gc count in log.

Additionally, good to have below enhancements for GenShen

> A potential enhancement for GenShen would be to provide used region counts for old and young separately, and to add the "Generational" prefix in:

`[2.067s][debug][gc,heap ] GC(11) Shenandoah Heap`

> Someday we should look at combining some of the other related messages (free set status, e.g.) into ShenandoahHeap::print_heap_on


[1]: https://github.com/openjdk/jdk/pull/26272#issuecomment-3063991526
[2]: https://github.com/openjdk/jdk/pull/26272#pullrequestreview-3012097755