[PATCH 7/9] perf stat: Add header for interval in JSON output

[Namhyung Kim <namhyung@kernel.org>]

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>
---
 tools/perf/util/stat-display.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index 25f67fb37f6d..aab2576bd40f 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -860,6 +860,10 @@ static void print_metric_headers(struct perf_stat_config *config,
 		if (!config->iostat_run)
 			fputs(aggr_header_csv[config->aggr_mode], config->output);
 	}
+	if (config->json_output) {
+		if (config->interval)
+			fputs("{\"unit\" : \"sec\"}", config->output);
+	}
 	if (config->iostat_run)
 		iostat_print_header_prefix(config);
 
-- 
2.38.1.431.g37b22c650d-goog