Resolve "Instrumentation for Deployment Approvals users"

What does this MR do and why?

Adds metric for tracking users who use deployment approvals feature

The metrics are named usage_activity_by_stage.release.users_creating_deployment_approvals and usage_activity_by_stage_monthly.release.users_creating_deployment_approvals

Metric Database Performance

SELECT COUNT(DISTINCT user_id) from deployment_approvals on production executed in 3.7ms and uses an existing index, which is well within recommended performance.

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/9643/commands/34190

How to set up and validate locally

Check the usage ping to see the new metrics

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 #355689 (closed)