SERVER-80769 moved several metrics (notably HelloMetrics) from ServiceContext to SessionManager, but due to loose lifecycle management of `shared_ptr<Session>` and the lack of requirement for a TransportLayer in unit tests which using replication logic, it's currently impossible to guarantee a SessionManager is available in all contexts which wish to mutate HelloMetrics.

Resolve this by tightening up transport object ownership, particularly around the Session objects. This will also allow removing the dummy "transportlessHelloMetrics" container for when (during unit tests) Sessions are unassociated to SessionManagers.