Add code to perf list to print metric groups, and metrics
that don't have an event name. The metricgroup code collects
the eventgroups and events into a rblist, and then prints
them according to the configured filters.

The metricgroups are printed by default, but can be
limited by perf list metric or perf list metricgroup

  % perf list metricgroup
  ..
  Metric Groups:

  DSB:
    DSB_Coverage
          [Fraction of Uops delivered by the DSB (aka Decoded Icache; or Uop Cache)]
  FLOPS:
    GFLOPs
          [Giga Floating Point Operations Per Second]
  Frontend:
    IFetch_Line_Utilization
          [Rough Estimation of fraction of fetched lines bytes that were likely consumed by program instructions]
  Frontend_Bandwidth:
    DSB_Coverage
          [Fraction of Uops delivered by the DSB (aka Decoded Icache; or Uop Cache)]
  Memory_BW:
    MLP
          [Memory-Level-Parallelism (average number of L1 miss demand load when there is at least 1 such miss)]

v2: Check return value of asprintf to fix warning on FC26
Fix key in lookup/addition for the groups list
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-8-andi@firstfloor.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>