perf stat: Fix trailing comma when there is no metric unit

Now that printing metric-value and metric-unit is optional,
print_running_json() shouldn't add the comma in case it becomes
trailing.

Replace all manual JSON comma stuff with a json_out() function that uses
the existing os->first tracking and auto inserts a comma if it's needed.
Update the test to handle that two of the fields can be missing.

This fixes the following test failure on Cortex A57 where the branch
misses metric is missing a required event:

  $ perf test -vvv "json output"

  106: perf stat JSON output linter:
  --- start ---
  test child forked, pid 665682
  Checking json output: no args Test failed for input:

  {"counter-value" : "3112.000000", "unit" : "",
   "event" : "armv8_pmuv3_1/branch-misses/",
   "event-runtime" : 20699340, "pcnt-running" : 100.00, }
  ...
  json.decoder.JSONDecodeError: Expecting property name enclosed in
  double quotes: line 12 column 144 (char 2109)
  ---- end(-1) ----
  106: perf stat JSON output linter                 : FAILED!

Fixes: e1cc918b ("perf stat: Drop metric-unit if unit is NULL")
Signed-off-by: James Clark <james.clark@linaro.org>
Tested-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20241112160048.951213-2-james.clark@linaro.org


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