This CL does 2 things:
1) Implements forwarding of histogram reporting from cppgc to v8 via
CppHeap.
2) Establishes the pipeline in GCTracer for sending the histograms to
the embedder.

Currently only cppgc histograms are populated.

See crrev.com/c/2916956 for usage.

Bug: chromium:1154636
Change-Id: I8150116f757e105d0dfac96a3f6e7dd95717f5bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2917033
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74830}

Some fixes that were required to make the metric recording framework run
better:
  - Set the foreground task runner later so it can still be modified in
    test cases
  - Add Start and Stop methods to TimedScope for more control
  - Clear map of contexts explicitly to avoid it being triggered at the
    end of the destructor when counters are already destroyed and a
    SEGFAULT may occur due to histogram updates during destruction of
    the weak persistent handles.

R=rmcilroy@chromium.org

Bug: chromium:1101749
Change-Id: Ib41c7aeb1aac96f0fa102f0fceadbf7ec2dd78dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2351668Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Emanuel Ziegler <ecmziegler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69422}

Add a framework for collecting event-based metrics like UKMs in V8
that is independent of the actual implementation.

Design doc: https://docs.google.com/document/d/1vCZQCh4B05isqwJOwTPv7WqcnVp4KJITMgsHSBg35ZI/

R=ulan@chromium.org

Bug: chromium:1101749
Change-Id: If3a5b954d1f0bcee4e06a03467b651feae378a5f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2288231
Commit-Queue: Emanuel Ziegler <ecmziegler@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69098}