RFR: 7903368: JMH: GC profiler misreports allocation and churn rates

[Aleksey Shipilev]

Look at the new test. If you multiply the allocation rate from `gc.alloc.rate.norm` by the benchmark throughput, it would disagree with `gc.alloc.rate`. This is because `gc.alloc.rate` is computed by GC profiler using its own timer, which accidentally includes the artificial delay before deregistering the GC notifications.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/87/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=87&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903368
  Stats: 81 lines in 2 files changed: 80 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jmh/pull/87.diff
  Fetch: git fetch https://git.openjdk.org/jmh pull/87/head:pull/87

PR: https://git.openjdk.org/jmh/pull/87