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

[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

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Commit messages:
 - Refill/direct allocation stats; fix test regex

Changes: https://git.openjdk.org/jdk/pull/9626/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9626&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290966
  Stats: 61 lines in 9 files changed: 41 ins; 0 del; 20 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