Remove read only repository metrics collector

Scrapping metrics from the database should be extracted into a separate deployment as it prevent horizontal scaling of the praefect instances. As part of praefect it could produce too much pressure on the database if we begin to scale it out. It also will produce too many metrics and create additional pressure on the collector. The solution for that problem is usage of additional exporter that will be deployed independently from the praefect instance.

Part of: #3286 (closed)