[OOBE][Perks] - Add metrics to the perks discovery screen

Add UMA histogram collection for the perks that we show in OOBE.
The following histograms are introduced in the CL:

1) OOBE.PerksDiscoveryScreen.Shown.{PerksID} - collects info
about each perk that was actually rendered in OOBE.
2) OOBE.PerksDiscoveryScreen.Selected.{PerksID} - whether
perk was selected/not selected by a user.
3) OOBE.PerksDiscoveryScreen.SelectedPerksCount - number of perks
selected.
4) OOBE.PerksDiscoveryScreen.SelectedPerksPercentage - ratio of
selected perks to their total amount.

Histograms are updated after user finished selecting all of the perks.

Bug: b/347181006
Change-Id: Iaa95007d438f0195be776f1007cdbf40f17d8d78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5769115
Commit-Queue: Bohdan Tyshchenko <bohdanty@google.com>
Reviewed-by: Osama Fathy <osamafathy@google.com>
Reviewed-by: Brahim Chikhaoui <bchikhaoui@google.com>
Cr-Commit-Position: refs/heads/main@{#1339777}