Ensure that duoProAssignedUsersCount accounts for enterprise seats too

Problem

Currently duoProAssignedUsersCount only counts seats for Duo Pro. We need to account for Enterprise too.

Solution

Update https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/analytics/ai_analytics/ai_metrics_service.rb#L28-35 to account for Duo Enterprise too.

Out of scope

duoProAssignedUsersCount will be update to duoAssignedUsersCount in a separate issue.