perf stat: Fix JSON output in metric-only mode

It generated a broken JSON output when aggregation mode or cgroup is
used with --metric-only option.  Also get rid of the header line and
make the output single line for each entry.

It needs to know whether the current metric is the first one or not.
So add 'first' field in the outstate and mark it false after printing.

Before:

  # perf stat -a -j --metric-only true
  {"unit" : "GHz"}{"unit" : "insn per cycle"}{"unit" : "branch-misses of all branches"}
  {{"metric-value" : "0.797"}{"metric-value" : "1.65"}{"metric-value" : "0.89"}
   ^

  # perf stat -a -j --metric-only --per-socket true
  {"unit" : "GHz"}{"unit" : "insn per cycle"}{"unit" : "branch-misses of all branches"}
  {"socket" : "S0", "aggregate-number" : 8, {"metric-value" : "0.295"}{"metric-value" : "1.88"}{"metric-value" : "0.64"}
                                           ^

After:

  # perf stat -a -j --metric-only true
  {"GHz" : "0.990", "insn per cycle" : "2.06", "branch-misses of all branches" : "0.59"}

  # perf stat -a -j --metric-only --per-socket true
  {"socket" : "S0", "aggregate-number" : 8, "GHz" : "0.439", "insn per cycle" : "2.14", "branch-misses of all branches" : "0.51"}

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/20221123180208.2068936-14-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>