[Autofill Assistant] Keep UMA metrics until M-93.

Bug: 1165009
Change-Id: I559011e1f24428663dbb2f25a7ceedb8945048ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627522


Commit-Queue: Clemens Arbesser <arbesser@google.com>
Auto-Submit: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849090}