[jdk17u-dev] RFR: 8307346: Add missing gc+phases logging for ObjectCount(AfterGC) JFR event collection code

[Oli Gillespie]

Backport for 8307346

This would be clean, except 8280450 adds some inconsequential code immediately after the hunks in g1 and parallel for reporting task queue stats, which causes a trivial conflict.

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Commit messages:
 - Backport 3f1927a7f3a2914402a25335c47a5a8bdd5511a6

Changes: https://git.openjdk.org/jdk17u-dev/pull/1344/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307346
  Stats: 12 lines in 3 files changed: 9 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1344.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1344/head:pull/1344

PR: https://git.openjdk.org/jdk17u-dev/pull/1344