Add enum for metadata entry source to 3PC mitigations deployment UKM.

The DT is no longer valid but we are repurposing this existing UKM event to track the user impact from various metadata sources (and decide on the future direction of 3PC global metadata).

UKM approval: https://docs.google.com/document/d/1Cal0aZiK79jgUwXLq3mY3FJjOViW92jipX0QWccR5Pg/edit?usp=sharing

Change-Id: I55c9e1352d5adaecc0ba5ad0b011fa7ac3d99005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6574802
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Auto-Submit: Anton Maliev <amaliev@chromium.org>
Commit-Queue: Anton Maliev <amaliev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1465962}