perf stat: Fix multi-line metric output in JSON

When a metric produces more than one values, it missed to print the opening
bracket.

Fixes: ab6baaae ("perf stat: Fix JSON output in metric-only mode")
Reported-by: Weilin Wang <weilin.wang@intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Weilin Wang <weilin.wang@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.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: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221202190447.1588680-1-namhyung@kernel.org


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