cppgc: Align cppgc::MetricRecorder with v8::metrics::Recorder

Update the MetricRecorder struct to have a similar structure and similar
fields to the GC struct defined in v8-metrics.h

Bug: chromium:1056170
Change-Id: Ieb77412476353557134aeac5cf8f66ab257e22dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2653236


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72394}