MEDIUM: counters: Dynamically allocate per-thread group counters

MEDIUM: counters: Dynamically allocate per-thread group counters

Instead of statically allocating the per-thread group counters,
based on the max number of thread groups available, allocate
them dynamically, based on the number of thread groups actually
used. That way we can increase the maximum number of thread
groups without using an unreasonable amount of memory.