MEDIUM: counters: return aggregate extra counters in ->fill_stats()

MEDIUM: counters: return aggregate extra counters in ->fill_stats()

Now thanks to new macro EXTRA_COUNTERS_AGGR() we can iterate over all
thread groups storages when returning the data for a given metric. This
remains convenient and mostly transparent. The caller continues to pass
the pointer to the metric in the first group, and offsets are calculated
for all other groups and data summed. For now all groups except the
first one contain only zeroes but reported values are nevertheless
correct.