Add JMX metrics mappings for Generational Shenandoah

[krrg]

GenShen was released with OpenJDK 24 and creates memory pools with the names Shenandoah Young Gen and Shenandoah Old Gen (see https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahMemoryPool.cpp#L102)

This adds mappings for those metrics so they can eventually be pulled all the way into DataDog.

[carlosroman]

So these metrics will be generated by default on JDK21 or do I need to enable a particular GC?

[krrg]

These metrics will only be generated with the Generational Shenandoah GC enabled.

For JDK 21, GenShen was gated behind an experimental flag, so you'd need something like

-XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational

For JDK 24+, GenShen is the default variant of Shenandoah, so you'd just need

-XX:+UseShenandoahGC

The metrics won't get mapped if the MBean doesn't exist (and it is created by enabling a specific GC, in this case Shenandoah). So if you do not have ShenandoahGC enabled, then the mapping would just be ignored, just like any of the other GC mappings for G1, ZGC, etc, if they are not in use, since the MBeans would be missing.

A similar change was made for Generational mode ZGC in #566.