[Recent Sessions] Add Monthly Active Days metric.

This records the number of active days in the past month (defined as
28 days to be consistent across months). This count works back from
the current day and is recorded whenever a new user education session
is started.

Bug: 341793283
Change-Id: I90db65be6db2ec40219466ae6681682f312cdff9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5550358
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1303988}