Pause migration when patroni apdex dropped below SLO for Gitlab.com

Extracted from #353395 (comment 892991285)

Overview

1. Metric: Patroni apdex
2. Action: When apdex is below SLO, pause migration
3. Source: Prometheus (for both apdex and SLO - see link below)
4. Requires prometheus: Yes

The patroni apdex indicates the overall database health. When it drops below a configured SLO, we can use this as a strong signal to pause background migrations until it recovered.

• Grafana
• Thanos

Note: Implementation details can be found here