RFR: 8277397: ZGC: Add JFR event for temporary latency measurements

[Stefan Karlsson]

I often measure latencies and stalls using JFR events. I'd like to add an event that can be used for these ad-hoc measurements during development and debugging.

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Commit messages:
 - 8277397: ZGC: Add JFR event for temporary latency measurements

Changes: https://git.openjdk.java.net/jdk/pull/6454/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6454&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277397
  Stats: 55 lines in 6 files changed: 55 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6454.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6454/head:pull/6454

PR: https://git.openjdk.java.net/jdk/pull/6454