This CL fixes a bug in the units of the reported metrics for
V8.GC.Cycle.MainThread.Full.Incremental.Mark (ms instead of us).

It also reports incremental marking/sweeping metrics (both for the
unified heap and the C++ managed heap) only when incremental
marking/sweeping were used; otherwise, no zero values are reported.

Bug: chromium:1154636
Bug: chromium:1343507
Change-Id: Ibc0103ea62fa0eeb5f7184280c8514e99a5c21a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3768502Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81803}