RFR: 8303135: JFR: Log periodic events using periodic tag

[Erik Gahlin]

Could I have a review of PR that improves logging for periodic events. 

Testing: jdk/jdk/jfr, tier1, tier2

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.org/jdk/pull/12741/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303135
  Stats: 59 lines in 7 files changed: 42 ins; 2 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/12741.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12741/head:pull/12741

PR: https://git.openjdk.org/jdk/pull/12741