RFR: 8368015: Shenandoah: fix error in computation of average allocation rate

[Kelvin Nilsen]

We use bytes_allocated_since_gc_start() to compute allocation rates.  This leaves a blind spot, as our current implementation ignores allocations and the time period between the moment we begin GC and the first time we update the allocation rate following the start of GC.  When this happens, we typically find that the sampled number of allocations is smaller than the allocations that had accumulated by the time we triggered the start of the current GC cycle.

This PR adds tracking for that accounting gap.

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Commit messages:
 - fix whitespace
 - Account for gap allocations between gc cycles

Changes: https://git.openjdk.org/jdk/pull/27398/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27398&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368015
  Stats: 32 lines in 4 files changed: 25 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27398.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27398/head:pull/27398

PR: https://git.openjdk.org/jdk/pull/27398