Collect metrics on how often plus addresses would suppress Autocomplete.

This CL attempts to simulate how often Autocomplete suggestions would
get suppressed by Plus Address suggestions if they were fully rolled
out. The metric will only be meaningful as long as only a negligible
portion of clients have Plus Addresses enabled and is therefore
targeted for removal later this year. This is reflected in the
expiry date of the metric.

Bug: 327328460
Change-Id: If110f9d7c7caf51843e84f5db143f19a66e37ed4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5332213


Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1267401}