Instrument Adoption for Custom Roles MVC

What does this MR do and why?

• Count of unique users with a custom role
• #382255 (closed)

DB Query

There isn't a lot of data on prod for this yet because it is a brand new feature but here is the generated sql for the metric in a 28 day timeframe (Gitlab::Usage::Metrics::Instrumentations::CountUsersWithCustomRolesMetric.new(time_frame: '28d', data_source: 'database').value).

 Gitlab::Usage::Metrics::Instrumentations::CountUsersWithCustomRolesMetric.new(time_frame: '28d', data_source: 'database').value
   (1.6ms)  SELECT MIN("members"."user_id") FROM "members" WHERE "members"."member_role_id" IS NOT NULL AND "members"."created_at" BETWEEN '2023-01-03 00:34:38.895132' AND '2023-01-31 00:34:38.895269' 

   (0.3ms)  SELECT MAX("members"."user_id") FROM "members" WHERE "members"."member_role_id" IS NOT NULL AND "members"."created_at" BETWEEN '2023-01-03 00:34:38.895132' AND '2023-01-31 00:34:38.895269' 

   (0.6ms)  SELECT COUNT(DISTINCT "members"."user_id") FROM "members" WHERE "members"."member_role_id" IS NOT NULL AND "members"."created_at" BETWEEN '2023-01-03 00:34:38.895132' AND '2023-01-31 00:34:38.895269' AND "members"."user_id" >= 0 AND "members"."user_id" < 1 
=> 0

And here is a database plan for

SELECT COUNT(DISTINCT "members"."user_id") FROM "members" WHERE "members"."member_role_id" IS NOT NULL

• https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/15100/commands/52461
• https://explain.depesz.com/s/7KD6

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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