RFR: metrics: add metrics for HotSpot

[Erik Helin]

Hi all,

please review this patch that adds some default metrics for HotSpot to the default `CollectorRegistry` in the `metrics` package.

Thanks,
Erik

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

Commit messages:
 - metrics: add metrics for HotSpot

Changes: https://git.openjdk.java.net/skara/pull/1160/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1160&range=00
  Stats: 125 lines in 5 files changed: 100 ins; 0 del; 25 mod
  Patch: https://git.openjdk.java.net/skara/pull/1160.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1160/head:pull/1160

PR: https://git.openjdk.java.net/skara/pull/1160