UMAs for metadata signal to host: exclude non-primary frame tree

While the last UMA we added shows a positive correlation between the
missing metadata signal case and short-lived RWHs, we have about 4x
other cases w/o RWH lifespan data.  We believe most of those cases
are for non-primary frames (where `compositor_metric_recorder_` is
null) but those cases also include primary frames that never received
commit signals.

This CL updates the UMA to exclude non-primary frames so that we can
confirm that missing the commit signal case is rare.

Bug: 40057499
Change-Id: I725f790682ba56bc03145aaaa3a349658c7a7aca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6253186
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Auto-Submit: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418959}