Metric for the number of read-only repositories

Adds a Prometheus collector for RepositoryStore to expose the number of read-only repositories within a virtual storage.

Since the state of the repositories is in the database, the current approach leads to redundant work as each Praefect will query the database for the number of read-only repositories. This could be fixed later by extracting an exporter from the metrics that use the database as their source of truth.

Alternatively, synchronization between Praefect nodes could be added in order to coordinate which Praefect should perform the potentially expensive queries for metrics.

Related to #2998 (closed)