Add number of users per access level in the usage ping
Description
Usage ping contains the number of users but we don't know the repartition between the different roles (Guest, Developer, ...).
To help us better understand how GitLab is used and make better decisions, we need to know how many users are in each access level.
Proposal
Add 5 new items in the usage ping
user_guest_level_count
user_reporter_level_count
user_developer_level_count
user_master_level_count
user_owner_level_count