RFR: 8290966: G1: Record number of PLAB filled and number of direct allocations [v2]

[Thomas Schatzl]

> Hi all,
> 
>   for evaluation in [JDK-8288966](https://bugs.openjdk.org/browse/JDK-8288966) I added statistics output that show the amount of PLAB fills and direct allocations; I think this is useful for similar evaluations in the future, so I kept and split it out from that change. Adds these values to the existing JFR event too.
> 
> Testing: PLAB related tests, gha
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  kbarrett review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9626/files
  - new: https://git.openjdk.org/jdk/pull/9626/files/7b242dd1..6194eea8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9626&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9626&range=00-01

  Stats: 15 lines in 1 file changed: 10 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/9626.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9626/head:pull/9626

PR: https://git.openjdk.org/jdk/pull/9626