Migrate k8s api unique user metrics to internal tracking

What does this MR do and why?

Part of #428008 (closed)

This MR migrates k8s_api unique users metrics to the new internal tracking. This should not change anything for the metrics, but this is the first step for the further improvements to the metrics. We will use the new internal tracking to add project information, so that we can get unique users for each project. We will create an another MR for this improvement.

It follows the steps: Migrating from tracking with RedisHLL

How to set up and validate locally

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

1. gdk start && gdk stop gitlab-k8s-agent
2. Run gitlab agent locally
3. Execute k8s api proxy requests with User / CI / PAT access (I tested with PAT access)
4. Check if Gitlab handles API requests to /usage_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.