[jdk8u-dev] RFR: 8266391: Replace use of reflection in jdk.internal.platform.Metrics

[Jonathan Dowland]

A clean backport of 8266391 for jdk8u-dev as part of cgroups v2 support.

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/176

Commit messages:
 - 8266391: Replace use of reflection in jdk.internal.platform.Metrics

Changes: https://git.openjdk.org/jdk8u-dev/pull/177/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=177&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8266391
  Stats: 103 lines in 4 files changed: 93 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/177.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/177/head:pull/177

PR: https://git.openjdk.org/jdk8u-dev/pull/177