Add metrics for projects with applied scan result policies

What does this MR do and why?

This MR adds metrics to count The total number of projects with at least one scan result policy.

Queries

SELECT COUNT(DISTINCT "approval_project_rules"."project_id") FROM "approval_project_rules" WHERE "approval_project_rules"."report_type" = 4 AND "approval_project_rules"."project_id" BETWEEN 268964 AND 278964;

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12479/commands/44104

Time: 63.098 ms
  - planning: 1.235 ms
  - execution: 61.863 ms
    - I/O read: 61.192 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)