RFR: 8258431: Provide a JFR event with live set size estimate [v12]

[Jaroslav Bachorik]

On Mon, 15 Mar 2021 21:22:44 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> For the liveness value to be useful it would have to be updated at each GC, and we need to investigate further to see how we can do that in a "cheap" way. I would prefer if G1 did just return used() in live() as a start and we can create a follow-up task to investigate how to best add a better estimate. Do you see any problem with this?

Actually yes - this event was meant to be a cheap way to see whether the *known* live size is growing (has an upwards trend) which would using the `used()` value make more difficult and unreliable. 

I would prefer keeping the implementation to return the lower bound of the live size as is the case for other GCs as well. May add explanatory comments to the code and the event definition to make this clear.

Also, the `used()` value is already captured in the event so we would have it duplicated.

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PR: https://git.openjdk.java.net/jdk/pull/2579