[repo] move repo stats to Service (closes #2951067) We currently add a new method on repo that anyone can call. Call service allows to reach the same result with: * Restricted access to manager only (thanks to selection on services), * Less unrelated code on the repo. * No need to fetch a reference to a repo object from client side. `req._cnx.repo` is not an API and config.repository() is on its way out. The old way to access this information (repo.stats()) is deprecated.