RFR: 8337501: JFR: Use TimespanUnit

[Erik Gahlin]

Could I have a review of change that updates the JFR code to use the newly introduced TimespanUnit enum. The enum is updated to use j.u.c.TimeUnit enum so the method TimeUnit::toNanos can be leveraged. It can handle over- and underflow properly.  

Path forward is to introduce a Timespan class, similar to the Rate class, that will prevent negative values, parse "infinity" and provide means to compare values. This will consolidate code further.


Testing: test/jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Move initialization in constructor
 - Initial

Changes: https://git.openjdk.org/jdk/pull/20392/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20392&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337501
  Stats: 57 lines in 4 files changed: 10 ins; 22 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/20392.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20392/head:pull/20392

PR: https://git.openjdk.org/jdk/pull/20392