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 <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

0 files changed, 0 insertions, 0 deletions