perf stat: Add header for interval in JSON output
It missed to print a matching header line for intervals.
Before:
# perf stat -a -e cycles,instructions --metric-only -j -I 500
{"unit" : "insn per cycle"}
{"interval" : 0.
500544283}{"metric-value" : "1.96"}
^C
After:
# perf stat -a -e cycles,instructions --metric-only -j -I 500
{"unit" : "sec"}{"unit" : "insn per cycle"}
{"interval" : 0.
500515681}{"metric-value" : "2.31"}
^C
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/20221107213314.3239159-8-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>