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: NKim Phillips <kim.phillips@amd.com>
Reviewed-by: NAndi Kleen <ak@linux.intel.com>
Tested-by: NArnaldo 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.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>