RFR: metrics: add process metrics

[Erik Helin]

Hi all,

please review this patch that adds the [Prometheus recommended](https://prometheus.io/docs/instrumenting/writing_clientlibs/#process-metrics) `process_*` metrics. I've tested the implementation manually on a Linux x64 machine and opted to start by only implementing support for all metrics on Linux hosts.

Thanks,
Erik

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Commit messages:
 - metrics: add process metrics

Changes: https://git.openjdk.java.net/skara/pull/1175/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1175&range=00
  Stats: 116 lines in 5 files changed: 89 ins; 0 del; 27 mod
  Patch: https://git.openjdk.java.net/skara/pull/1175.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1175/head:pull/1175

PR: https://git.openjdk.java.net/skara/pull/1175