RFR: 8325574: Shenandoah: Simplify and enhance reporting of requested GCs [v3]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add comments to new APIs

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17795/files
  - new: https://git.openjdk.org/jdk/pull/17795/files/9115c470..8d174e81

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17795&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17795&range=01-02

  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 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