MINOR: counters: store a tgroup step for extra_counters to access multiple tgroups

MINOR: counters: store a tgroup step for extra_counters to access multiple tgroups

We'll need to permit any user to update its own tgroup's extra counters
instead of the global ones. For this we now store the per-tgroup step
between two consecutive data storages, for when they're stored in a
tgroup array. When shared (e.g. resolvers or listeners), we just store
zero to indicate that it doesn't scale with tgroups. For now only the
registration was handled, it's not used yet.