8350854: Include thread counts in safepoint logging

[pengxiaolong]

The PR is to add thread counts in safepoint logging. In recent work, we notice the number of Java thread may affects time to reach/leave safepoint hardly in some cases, but safepoint logging doesn't have thread counts, thread counts are printed only with 'safepoint+stats' log.

Given in many cases, we only have safepoint log enabled, not safepoint+stats, it make sense to add thread count to 'safepoint' logging.

Here is example log output with the change:

[15.240s][info][safepoint      ] Safepoint "ShenandoahFinalMarkStartEvac", Time since last: 3527459 ns, Reaching safepoint: 78666 ns, At safepoint: 821625 ns, Leaving safepoint: 1472167 ns, Total: 2372458 ns, Threads: 3 runnable, 1038 total

Test

• Tire1
• Tire2

---

Progress

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Issue

• JDK-8350854: Include thread counts in safepoint logging (Enhancement - P4)

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