Move alerts on system & custom metrics and feature into GitLab Core

This is an implementation issue to move our alerts on system & custom metrics within the Monitor stage into Core.

• System: Metrics that are displayed in the system dashboard (common_metrics.yml).
• Custom: Metric at the bottom of the system dashboard (Business, System, ...)

Implementation

Here are files that use the code of custom metrics and alerts. They live at the intersection of the Custom Metrics and Alerting features. Once both features have been moved to Core, we can move these files into Core as well.

1. EE::Gitlab::Metrics::Dashboard::Stages::AlertsInserter
2. Files in ee/app/assets/javascripts/monitoring/

Dependencies:

• Custom Metrics: #42639 (closed)
• Alerts: #42640 (closed)