RFR: 8325574: Shenandoah: Simplify and enhance reporting of requested GCs

[William Kemper]

Shenandoah distinguishes between 'implicit' and 'explicit' GC requests. The distinction is used to decide whether or not a request is serviced concurrently or with a full GC. The data collected is also included in the end-of-process report. This change simplifies handling of these requests and adds a tally of the underlying GC causes to the end-of-process report.

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Commit messages:
 - Simplify handling/reporting of implicit/explicit requested GCs

Changes: https://git.openjdk.org/jdk/pull/17795/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17795&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325574
  Stats: 109 lines in 5 files changed: 44 ins; 42 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/17795.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17795/head:pull/17795

PR: https://git.openjdk.org/jdk/pull/17795