[jdk21u] RFR: 8318474: Fix memory reporter for thread_count

[Evgeny Ignatenko]

Clean backport of [JDK-8318474](https://bugs.openjdk.org/browse/JDK-8318474)

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Commit messages:
 - Backport 0baa9ecd76c9be33a1939e47dcae320bc8b65e96

Changes: https://git.openjdk.org/jdk21u/pull/311/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318474
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/311.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/311/head:pull/311

PR: https://git.openjdk.org/jdk21u/pull/311