Add shouty warning about misusing postgres_exporter for gitlab system monitoring

This file is for PostgreSQL statistics exporting. Keep in mind any metrics exported from here will be:

1. exported from every database including read-only replicas
2. scraped frequently based on database-monitoring needs
3. Queried live on demand for every scrape

Moreover this file does not have tests and if you get this file wrong you can easily disable all database monitoring metrics.

If you want to monitor the Gitlab system as a whole you're almost certainly looking for gitlab-monitor:

https://gitlab.com/gitlab-org/gitlab-monitor

There you will be able to write Ruby code, cache data, access Redis, etc.