Measures the time to load data keys for Attribution Reporting API

This may help us determine whether optimization is needed for the
database to make the API more efficient.

Change-Id: I7838d97c9b954c6e18ded471482f4e962b6aaddd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6197384
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1414105}