RFR: 8302677: JFR: Cache label and contentType in EventType and ValueDescriptor

[Erik Gahlin]

Could I have a review of change that caches getLabel() and getContentType().

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Remove patch.txt
 - Initial

Changes: https://git.openjdk.org/jdk/pull/12603/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12603&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302677
  Stats: 24 lines in 2 files changed: 14 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12603.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12603/head:pull/12603

PR: https://git.openjdk.org/jdk/pull/12603