framemetrics enhancements

Mostly improvements to better support amd perfcntrs:

1. allow specifying individual counters within groups, since most drivers/hw supporting the amd perfcntr extension have more things that can be counted than actual counters
2. refactor to allow multiple counter groups, since most drivers/hw supporting amd perfcntr extension can capture counters from different groups with the same query

I tried as much as possible to not change the intel perfcntr case, mostly because I'm not super clear about that extension and I didn't want to break things. AFAICT it maybe supports collecting counters from different groups at the same time, but by using a different query? In which case it should be possible to refactor the intel counters to support this too, but keeping the query in the PerfCounterGroup object, rather than PerfCounters object as is done with the amd case.