perf stat: Do not indent headers for JSON

Currently --metric-only with --json indents header lines.  This is not
needed for JSON.

  $ perf stat -aA --metric-only -j true
        {"unit" : "GHz"}{"unit" : "insn per cycle"}{"unit" : "branch-misses of all branches"}
  {"cpu" : "0", {"metric-value" : "0.101"}{"metric-value" : "0.86"}{"metric-value" : "1.91"}
  {"cpu" : "1", {"metric-value" : "0.102"}{"metric-value" : "0.87"}{"metric-value" : "2.02"}
  {"cpu" : "2", {"metric-value" : "0.085"}{"metric-value" : "1.02"}{"metric-value" : "1.69"}
  ...

Note that the other lines are broken JSON, but it will be handled later.

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/20221107213314.3239159-7-namhyung@kernel.org


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