RFR: 8367378: GenShen: Missing timing stats when old mark buffers are flushed during final update refs

[William Kemper]

Clean backport. Adds trivial instrumentation.

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Commit messages:
 - Backport 11df28916af4c2bfd69b2829c74e046ad953ee6c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/220/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=220&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367378
  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/220.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/220/head:pull/220

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/220