public page showing the amount of objects (content count et al.) in 3rd party mirrors

We want to publicly show (e.g., on the archive page) how many objects are available from 3rd party mirrors (e.g., the Azure mirror we're currently populating #691 (closed)).

This could be based on the data we already have from munin, we just need to project them to 3rd party mirrors, and do the needed integration with the website.