[jdk17u-dev] RFR: 8290966: G1: Record number of PLAB filled and number of direct allocations

[Ben Taylor]

Clean, low risk backport of a change to improve logging around full PLABs and direct allocations. 

Passes the included test, as well as `make test TEST=gc/g1` locally on linux x64.

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Commit messages:
 - Backport 30205bb289e9b25d60474b24b891e15923071b5a

Changes: https://git.openjdk.org/jdk17u-dev/pull/2777/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2777&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290966
  Stats: 73 lines in 9 files changed: 51 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2777.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2777/head:pull/2777

PR: https://git.openjdk.org/jdk17u-dev/pull/2777