Add metrics for groups with assigned security policy project

What does this MR do and why?

This MR adds metrics to count The total number of groups with an assigned security policy project.

Queries

SELECT COUNT(DISTINCT "security_orchestration_policy_configurations"."namespace_id") FROM "security_orchestration_policy_configurations" WHERE "security_orchestration_policy_configurations"."namespace_id" BETWEEN 10000 AND 20000;

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12479/commands/44106

Time: 20.750 ms
  - planning: 0.622 ms
  - execution: 20.128 ms
    - I/O read: 19.874 ms
    - I/O write: 0.000 ms

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #375284 (closed)