Replace History.Embeddings.Embedder.CpuUsage with CpuUsage2

This is logged every 30 sec instead of every 2 minutes, and finishes
recording an interval if the service process exits partway through.

To implement the new semantic, moves most of CpuHistogramLogger to an
inner class that's owned by CpuHistogramLogger and scheduled for
deletion at the end of the recording interval. CpuHistogramLogger now
exists for the whole lifetime of PassageEmbeddingsServiceController,
which starts and stops logging when the service process starts and
exits.

Also fixes a CHECK that can be hit in rare cases due to a race.

OBSOLETE_HISTOGRAM[History.Embeddings.Embedder.CpuUsage]=Replaced by History.Embeddings.Embedder.CpuUsage2

Bug: 359685658
Change-Id: Iffb8ba3b030aa14fa90bc8cd335663619aacd213
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6055322
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389437}