RFR: 8307346 - Add missing gc+phases logging for ObjectCount(AfterGC) JFR event collection code [v2]

[olivergillespie]

> Add logging for the time taken to collect/report ObjectCount(AfterGC) event to Serial, Parallel and G1 full collectors, for parity with the G1 concurrent collector. This event can be *very* expensive to collect (single threaded STW full heap scan), so it's very important to show it clearly to the user.
> 
> Added output:
> 
> Serial
> 
> [7.976s][debug][gc,phases      ] GC(7) Trigger cleanups 0.002ms
> [7.977s][debug][gc,phases      ] GC(7) Class Unloading 0.556ms
> ++ [7.977s][debug][gc,phases,start] GC(7) Report Object Count
> ++ [8.529s][debug][gc,phases      ] GC(7) Report Object Count 552.065ms
> [8.529s][info ][gc,phases      ] GC(7) Phase 1: Mark live objects 1066.882ms
> [8.529s][info ][gc,phases,start] GC(7) Phase 2: Compute new object addresses
> 
> Parallel
> 
> [5.786s][debug][gc,phases      ] GC(12) Trigger cleanups 0.002ms
> [5.786s][debug][gc,phases      ] GC(12) Class Unloading 0.556ms
> ++ [5.786s][debug][gc,phases,start] GC(12) Report Object Count
> ++ [6.313s][debug][gc,phases      ] GC(12) Report Object Count 526.307ms
> [6.313s][info ][gc,phases      ] GC(12) Marking Phase 889.900ms
> [6.313s][info ][gc,phases,start] GC(12) Summary Phase
> 
> G1 Full
> 
> [3.922s][debug][gc,phases         ] GC(24) Trigger cleanups 0.002ms
> [3.922s][debug][gc,phases         ] GC(24) Phase 1: Class Unloading and Cleanup 0.159ms
> ++ [3.922s][debug][gc,phases,start   ] GC(24) Report Object Count
> ++ [4.442s][debug][gc,phases         ] GC(24) Report Object Count 519.292ms
> [4.442s][info ][gc,phases         ] GC(24) Phase 1: Mark live objects 653.209ms
> [4.442s][info ][gc,phases,start   ] GC(24) Phase 2: Prepare compaction

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

  Use correct holder var names

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13772/files
  - new: https://git.openjdk.org/jdk/pull/13772/files/3ef4f4cc..ce7227c5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13772&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13772&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13772.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13772/head:pull/13772

PR: https://git.openjdk.org/jdk/pull/13772