[crac] RFR: 8373687: [CRaC] Add metrics through jcmd [v6]

[Radim Vansa]

> Enable user adding custom metrics (and labels) through jcmd JDK.checkpoint command, e.g. using
> 
> jcmd <pid> JDK.checkpoint metrics=foo=123,bar=0.456

Radim Vansa has updated the pull request incrementally with one additional commit since the last revision:

  Address review

-------------

Changes:
  - all: https://git.openjdk.org/crac/pull/281/files
  - new: https://git.openjdk.org/crac/pull/281/files/919393d0..2f595fdd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=281&range=05
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=281&range=04-05

  Stats: 100 lines in 7 files changed: 42 ins; 45 del; 13 mod
  Patch: https://git.openjdk.org/crac/pull/281.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/281/head:pull/281

PR: https://git.openjdk.org/crac/pull/281