client/network/service: Add primary dimension to connection metrics

Two nodes can be interconnected via one or more connections. The first
of those connections is called the primary connection.

This commit adds another dimension to the
`sub_libp2p_connections_{closed,opened}_total` metrics to differentiate
primary and non-primary connections being opened / closed.

By intuition more than one connection between two nodes is rare.
Tracking the fact whether a connection is primary or not will help prove
or disprove this intuition.