perf stat: Fix summary output in CSV with --metric-only

It should not print "summary" for each event when --metric-only is set.

Before:

  $ sudo perf stat -a --per-socket --summary -x, --metric-only true
   time,socket,cpusGhz,insn per cycle,branch-misses of all branches,
       0.000709079,S0,8,0.893,2.40,0.45,
  S0,8,         summary,         summary,         summary,         summary,         summary,0.893,         summary,2.40,         summary,         summary,0.45,

After:

  $ sudo perf stat -a --per-socket --summary -x, --metric-only true
   time,socket,cpusGHz,insn per cycle,branch-misses of all branches,
       0.000882297,S0,8,0.598,1.64,0.64,
           summary,S0,8,0.598,1.64,0.64,

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221112032244.1077370-11-namhyung@kernel.org


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