perf stat: Tidy up JSON metric-only output when no metrics

It printed empty strings for each metric.  I guess it's needed for CSV
output to match the column number.  We could just ignore the empty
metrics in JSON but it ended up with a broken JSON object with a
trailing comma.

So I added a dummy '"metric-value" : "none"' part.  To do that, it
needs to pass struct outstate to print_metric_end() to check if any
metric value is printed or not.

Before:

  # perf stat -aj --metric-only --per-socket --for-each-cgroup system.slice true
  {"socket" : "S0", "cpu-count" : 8, "cgroup" : "system.slice", "" : "", "" : "", "" : "", "" : "", "" : "", "" : "", "" : "", "" : ""}

After:

  # perf stat -aj --metric-only --per-socket --for-each-cgroup system.slice true
  {"socket" : "S0", "cpu-count" : 8, "cgroup" : "system.slice", "metric-value" : "none"}

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.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/20221123180208.2068936-16-namhyung@kernel.org


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