metrics: Total amount of connections to the service

The existing metric 'gitaly_connections_total' exists on a package level (the global metric) and used for both praefect and gitaly services. With this change we address both those problems: the metric now are local to the service and a new metric introduced specially for praefect following the same naming pattern as all other praefect-related metrics.

The content of the connectioncounter package moved into a starter package as it makes no sense to have it separately.

Change: added