Fix for an implementation with non-contiguous group ID values.

Fix for an implementation with non-contiguous group ID values.

It seems crazy to me that group IDs (being integers) can be anything
other than [0 .. num_groups - 1], but the specification is written
with full generality here.

The code was already querying the group ID values originally, and
assigning those to each group->id slot in the metric_group_info_t
structure. But after that, the code had been assuming it could just
use values from 0 .. num_groups-1.

Fix this by carefully using group_index values ([0..num_groups-1])
when indexing into the various arrays and group->id values when
passing ID values to the various performance-monitor API functions.