Thresholds flush for the json output

The JSON Output implements WithThresholds
so the Engine can set the thresholds invoking SetThresholds.

The flush logic sets the Metric's Thresholds
to write them in the json output.

Closes #1052

output/json/json.go

@@ -32,7 +32,7 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-// TODO: add option for emitting proper JSON files (https://github.com/loadimpact/k6/issues/1052)
+// TODO: add option for emitting proper JSON files (https://github.com/loadimpact/k6/issues/737)
 const flushPeriod = 200 * time.Millisecond // TODO: make this configurable
 
 // Output funnels all passed metrics to an (optionally gzipped) JSON file.
@@ -47,6 +47,7 @@ type Output struct {
 	encoder     *stdlibjson.Encoder
 	closeFn     func() error
 	seenMetrics map[string]struct{}
+	thresholds  map[string][]*stats.Threshold
 }
 
 // New returns a new JSON output.
@@ -118,6 +119,15 @@ func (o *Output) Stop() error {
 	return o.closeFn()
 }
 
+// SetThresholds receives the thresholds before the output is Start()-ed.
+func (o *Output) SetThresholds(thresholds map[string]stats.Thresholds) {
+	ths := make(map[string][]*stats.Threshold)
+	for name, t := range thresholds {
+		ths[name] = append(ths[name], t.Thresholds...)
+	}
+	o.thresholds = ths
+}
+
 func (o *Output) flushMetrics() {
 	samples := o.GetBufferedSamples()
 	start := time.Now()
@@ -127,6 +137,7 @@ func (o *Output) flushMetrics() {
 		count += len(samples)
 		for _, sample := range samples {
 			sample := sample
+			sample.Metric.Thresholds.Thresholds = o.thresholds[sample.Metric.Name]
 			o.handleMetric(sample.Metric)
 			err := o.encoder.Encode(WrapSample(sample))
 			if err != nil {

output/json/json_test.go

@@ -71,9 +71,8 @@ func generateTestMetricSamples(t *testing.T) ([]stats.SampleContainer, func(io.R
 		}, Time: time2, Tags: connTags},
 		stats.Sample{Time: time3, Metric: metric2, Value: float64(5), Tags: stats.NewSampleTags(map[string]string{"tag3": "val3"})},
 	}
-	// TODO: fix JSON thresholds (https://github.com/loadimpact/k6/issues/1052)
 	expected := []string{
-		`{"type":"Metric","data":{"name":"my_metric1","type":"gauge","contains":"default","tainted":null,"thresholds":[],"submetrics":null,"sub":{"name":"","parent":"","suffix":"","tags":null}},"metric":"my_metric1"}`,
+		`{"type":"Metric","data":{"name":"my_metric1","type":"gauge","contains":"default","tainted":null,"thresholds":["rate<0.01", "p(99)<250"],"submetrics":null,"sub":{"name":"","parent":"","suffix":"","tags":null}},"metric":"my_metric1"}`,
 		`{"type":"Point","data":{"time":"2021-02-24T13:37:10Z","value":1,"tags":{"tag1":"val1"}},"metric":"my_metric1"}`,
 		`{"type":"Point","data":{"time":"2021-02-24T13:37:10Z","value":2,"tags":{"tag2":"val2"}},"metric":"my_metric1"}`,
 		`{"type":"Metric","data":{"name":"my_metric2","type":"counter","contains":"data","tainted":null,"thresholds":[],"submetrics":null,"sub":{"name":"","parent":"","suffix":"","tags":null}},"metric":"my_metric2"}`,
@@ -94,6 +93,8 @@ func TestJsonOutputStdout(t *testing.T) {
 		StdOut: stdout,
 	})
 	require.NoError(t, err)
+
+	setThresholds(t, out)
 	require.NoError(t, out.Start())
 
 	samples, validateResults := generateTestMetricSamples(t)
@@ -130,6 +131,8 @@ func TestJsonOutputFile(t *testing.T) {
 		ConfigArgument: "/json-output",
 	})
 	require.NoError(t, err)
+
+	setThresholds(t, out)
 	require.NoError(t, out.Start())
 
 	samples, validateResults := generateTestMetricSamples(t)
@@ -156,6 +159,8 @@ func TestJsonOutputFileGzipped(t *testing.T) {
 		ConfigArgument: "/json-output.gz",
 	})
 	require.NoError(t, err)
+
+	setThresholds(t, out)
 	require.NoError(t, out.Start())
 
 	samples, validateResults := generateTestMetricSamples(t)
@@ -185,3 +190,15 @@ func TestWrapMetricWithMetricPointer(t *testing.T) {
 	out := wrapMetric(&stats.Metric{})
 	assert.NotEqual(t, out, (*Envelope)(nil))
 }
+
+func setThresholds(t *testing.T, out output.Output) {
+	t.Helper()
+
+	jout, ok := out.(*Output)
+	require.True(t, ok)
+
+	ts, err := stats.NewThresholds([]string{"rate<0.01", "p(99)<250"})
+	require.NoError(t, err)
+
+	jout.SetThresholds(map[string]stats.Thresholds{"my_metric1": ts})
+}