RFR: 8277450: Record number of references into collection set during gc

[Thomas Schatzl]

Hi all,

  could I have reviews for this change that adds logging for the number of heap roots found and pushed onto the task queues from the scanned cards? This can help with finding a good card size.

Thanks,
  Thomas

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Commit messages:
 - Added "Found Roots" metric to Scan Heap Roots logging

Changes: https://git.openjdk.java.net/jdk/pull/6519/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6519&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277450
  Stats: 19 lines in 6 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6519.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6519/head:pull/6519

PR: https://git.openjdk.java.net/jdk/pull/6519