JFR Event for live set size in ZGC

[Jaroslav Bachorík]

Hello,

I am trying to resurrect my PR (https://github.com/openjdk/jdk/pull/2579)
about adding a JFR event for live set size but in a less ambitious form.
Having this information in a JFR recording would be extremely useful for
Datadog but I am sure that other perf tools vendors might be interested in
this as well.

Instead of trying to add this event to all GC implementations I would like
to circle back and add this event to a single implementation at a time,
given that adding such an event would not be controversial or negatively
affect the GC implementation by doing significantly more work just in order
to compute the live set size.

I would like to start with ZGC where the live set size is already known and
quite easy to use without any additional computation.
>From my previous work on this problem it seems like adding this event for
Shenandoah, ParallelGC and Epsilon should also not create a lot of
controversy.

I am looking for feedback before investing more time in this effort - I
would really like to avoid the situation I experienced with my previous PR
when it was basically shot down after quite a lot of time spent on
developing and reviewing it.

Cheers,

-JB-
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