io_queue: Move metrics registration from class constructor
io_queue: Move metrics registration from class constructor

This will allow to move it further -- into the io_queue class (see
next patch).

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar