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

[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

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Commit messages:
 - 8373687: [CRaC] Add metrics through jcmd

Changes: https://git.openjdk.org/crac/pull/281/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=281&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373687
  Stats: 312 lines in 9 files changed: 298 ins; 0 del; 14 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