[AF] Fix placement of metrics enum warning

Follow-up CL from comment:
https://chromium-review.googlesource.com/c/chromium/src/+/6802601/comment/69831dbd_8736dbff/

Enum values for metrics that are persisted to logs must include a
specific comment:
https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histograms/README.md#requirements

However, it seems in many places the comment is getting attached to the
first enum value instead of the enum itself, which may be incorrect
reflected by some IDEs, potentially causing confusion.

This CL updates relevant files to follow the standard in the style
guide.

NO_IFTTT=Comment change only

Change-Id: Icf901db0a737b1b7b8ca1063c8f10de7df641364
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6820833
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Olivia Saul <osaul@google.com>
Cr-Commit-Position: refs/heads/main@{#1497049}