Support Shenandoah GC in JvmGcMetrics

[IRus]

Micrometer: 1.3.5
JDK: 11.0.6

Right now all metrics exported from JvmGcMetrics are 0.

---

See the OpenJDK Wiki for Shenandoah GC for more information on it. While the GC pause time metric (jvm.gc.pause) was working, we should also support the following:

• jvm.gc.max.data.size
• jvm.gc.live.data.size
• jvm.gc.memory.allocated

[asad-awadia]

And/Or metronome from openj9

[shakuzen]

@asad-awadia Could you open a separate issue for that? I did not know OpenJ9 had its own garbage collector.

[asad-awadia]

@shakuzen #1458 I think there is already an issue? Didn't want to make duplicates

Not sure if metronome is part of that ticket though