Add totalActiveUsers to subscriptionUsage.usersUsage

What does this MR do and why?

This adds totalActiveUsers to subscriptionUsage.usersUsage as part of https://gitlab.com/gitlab-org/gitlab/-/work_items/594756

Clients can now query how many users are using gitlab credits

Changelog: added EE: true

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/594756+

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Checkout this MR's steps to generate some data.
2. Open GraphQL and execute this query:

Query:

query getSubscriptionUsage($namespacePath: ID) {
  subscriptionUsage(namespacePath: $namespacePath) {
    usersUsage {
      totalActiveUsers
    }
  }
}

Variables:

{
  "namespacePath": "your-group"
}

MR acceptance checklist

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