perf list: Allow plurals for metric, metricgroup

Enhance usability by allowing the same plurality used in the output
title, for the command line parameter.

BEFORE, perf deceitfully acts as if there are no metrics to be had:

  $ perf list metrics

  List of pre-defined events (to be used in -e):

  Metric Groups:

  $

But singular 'metric' shows a list of metrics:

  $ perf list metric

  List of pre-defined events (to be used in -e):

  Metrics:

    IPC
         [Instructions Per Cycle (per logical thread)]
    UPI
         [Uops Per Instruction]

AFTER, when asking for 'metrics', we actually see the metrics get listed:

  $ perf list metrics

  List of pre-defined events (to be used in -e):

  Metrics:

    IPC
         [Instructions Per Cycle (per logical thread)]
    UPI
         [Uops Per Instruction]

Fixes: 71b0acce ("perf list: Add metric groups to perf list")
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Janakarajan Natarajan <janakarajan.natarajan@amd.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Luke Mujica <lukemujica@google.com>
Cc: Martin Liška <mliska@suse.cz>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20190919204306.12598-4-kim.phillips@amd.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>