RFR: JDK-8260571: Add PrintMetaspaceStatistics to print metaspace statistics upon VM exit

[Thomas Stuefe]

This patch adds a new diagnostic boolean switch `PrintMetaspaceStatistics`. When set, it will cause the VM to print a brief report about metaspace occupancy when exiting. It follows the same logic as `PrintNMTStatistics` and similar switches.

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/2283/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2283&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260571
  Stats: 11 lines in 2 files changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2283.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2283/head:pull/2283

PR: https://git.openjdk.java.net/jdk/pull/2283