RFR: 8291753: Add JFR event for GC CPU Time [v3]

[Sangheon Kim]

> Hi all,
> 
> Could I have reviews to add new JFR event for GC CPU time?
> Currently only log message is available for CPU time (user, system, real).
> As there is already GCTraceCPUTime class which is used for a log message, I added GCTracer to deliver the event.
> The log message of CPU time is printed after GC is completed and tried to keep same.
> 
> For G1, manually checked there is not difference.
> 
> For test, I had to add an exception as GCCpuTime will be generated after GC end.
> 
> Testing: tier 1 ~ 3
> 
> Thanks,
> Sangheon

Sangheon Kim has updated the pull request incrementally with one additional commit since the last revision:

  Reflect egahlin's comment

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9760/files
  - new: https://git.openjdk.org/jdk/pull/9760/files/2075927c..f4329777

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9760&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9760&range=01-02

  Stats: 28 lines in 7 files changed: 0 ins; 0 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/9760.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9760/head:pull/9760

PR: https://git.openjdk.org/jdk/pull/9760