When the threshold isn't unknown add a value to the json like:
"metric-threshold" : "good"

A more complete example:
```
$ perf stat -a -j -I 1000
{"interval" : 1.001089747, "counter-value" : "16045.281449", "unit" : "msec", "event" : "cpu-clock", "event-runtime" : 16045355135, "pcnt-running" : 100.00, "metric-value" : "16.045281", "metric-unit" : "CPUs utilized"}
{"interval" : 1.001089747, "counter-value" : "10003.000000", "unit" : "", "event" : "context-switches", "event-runtime" : 16045314844, "pcnt-running" : 100.00, "metric-value" : "623.423156", "metric-unit" : "/sec"}
{"interval" : 1.001089747, "counter-value" : "328.000000", "unit" : "", "event" : "cpu-migrations", "event-runtime" : 16045321403, "pcnt-running" : 100.00, "metric-value" : "20.442147", "metric-unit" : "/sec"}
{"interval" : 1.001089747, "counter-value" : "20114.000000", "unit" : "", "event" : "page-faults", "event-runtime" : 16045355927, "pcnt-running" : 100.00, "metric-value" : "1.253577", "metric-unit" : "K/sec"}
{"interval" : 1.001089747, "counter-value" : "4066679471.000000", "unit" : "", "event" : "instructions", "event-runtime" : 16045369123, "pcnt-running" : 100.00, "metric-value" : "1.628330", "metric-unit" : "insn per cycle"}
{"interval" : 1.001089747, "counter-value" : "2497454658.000000", "unit" : "", "event" : "cycles", "event-runtime" : 16045374810, "pcnt-running" : 100.00, "metric-value" : "0.155650", "metric-unit" : "GHz"}
{"interval" : 1.001089747, "counter-value" : "914974294.000000", "unit" : "", "event" : "branches", "event-runtime" : 16045379877, "pcnt-running" : 100.00, "metric-value" : "57.024509", "metric-unit" : "M/sec"}
{"interval" : 1.001089747, "counter-value" : "9237201.000000", "unit" : "", "event" : "branch-misses", "event-runtime" : 16045375017, "pcnt-running" : 100.00, "metric-value" : "1.009559", "metric-unit" : "of all branches", "metric-threshold" : "good"}
{"interval" : 1.001089747, "event-runtime" : 16045397172, "pcnt-running" : 100.00, "metricgroup" : "TopdownL1"}
{"interval" : 1.001089747, "metric-value" : "22.036686", "metric-unit" : "%  tma_backend_bound", "metric-threshold" : "bad"}
{"interval" : 1.001089747, "metric-value" : "7.610161", "metric-unit" : "%  tma_bad_speculation", "metric-threshold" : "good"}
{"interval" : 1.001089747, "metric-value" : "36.729687", "metric-unit" : "%  tma_frontend_bound", "metric-threshold" : "bad"}
{"interval" : 1.001089747, "metric-value" : "33.623465", "metric-unit" : "%  tma_retiring"}
...
```

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/tests/shell/lib/perf_json_output_lint.py | 5 ++++-
 tools/perf/util/stat-display.c                      | 9 +++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/tools/perf/tests/shell/lib/perf_json_output_lint.py b/tools/perf/tests/shell/lib/perf_json_output_lint.py
index abc1fd737782..8ddb85586131 100644
--- a/tools/perf/tests/shell/lib/perf_json_output_lint.py
+++ b/tools/perf/tests/shell/lib/perf_json_output_lint.py
@@ -57,6 +57,7 @@ def check_json_output(expected_items):
       'interval': lambda x: isfloat(x),
       'metric-unit': lambda x: True,
       'metric-value': lambda x: isfloat(x),
+      'metric-threshold': lambda x: x in ['unknown', 'good', 'less good', 'nearly bad', 'bad'],
       'metricgroup': lambda x: True,
       'node': lambda x: True,
       'pcnt-running': lambda x: isfloat(x),
@@ -68,13 +69,15 @@ def check_json_output(expected_items):
   for item in json.loads(input):
     if expected_items != -1:
       count = len(item)
-      if count != expected_items and count >= 1 and count <= 6 and 'metric-value' in item:
+      if count != expected_items and count >= 1 and count <= 7 and 'metric-value' in item:
         # Events that generate >1 metric may have isolated metric
         # values and possibly other prefixes like interval, core,
         # aggregate-number, or event-runtime/pcnt-running from multiplexing.
         pass
       elif count != expected_items and count >= 1 and count <= 5 and 'metricgroup' in item:
         pass
+      elif count == expected_items + 1 and 'metric-threshold' in item:
+          pass
       elif count != expected_items:
         raise RuntimeError(f'wrong number of fields. counted {count} expected {expected_items}'
                            f' in \'{item}\'')
diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index 5b5923730a7b..f699e39abcd8 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -496,15 +496,20 @@ static void print_metric_csv(struct perf_stat_config *config,
 
 static void print_metric_json(struct perf_stat_config *config __maybe_unused,
 			     void *ctx,
-			     enum metric_threshold_classify thresh __maybe_unused,
+			     enum metric_threshold_classify thresh,
 			     const char *fmt __maybe_unused,
 			     const char *unit, double val)
 {
 	struct outstate *os = ctx;
 	FILE *out = os->fh;
 
-	if (unit)
+	if (unit) {
 		fprintf(out, "\"metric-value\" : \"%f\", \"metric-unit\" : \"%s\"", val, unit);
+		if (thresh != METRIC_THRESHOLD_UNKNOWN) {
+			fprintf(out, ", \"metric-threshold\" : \"%s\"",
+				metric_threshold_classify__str(thresh));
+		}
+	}
 	if (!config->metric_only)
 		fprintf(out, "}");
 }
-- 
2.47.0.rc1.288.g06298d1525-goog