Fix export timestamp not working for prometheus on v2 (influxdata#7289)

Author: kir4h

plugins/outputs/prometheus_client/prometheus_client.go

@@ -139,7 +139,7 @@ func (p *PrometheusClient) Init() error {
 			return err
 		}
 	case 2:
-		p.collector = v2.NewCollector(p.ExpirationInterval.Duration, p.StringAsLabel)
+		p.collector = v2.NewCollector(p.ExpirationInterval.Duration, p.StringAsLabel, p.ExportTimestamp)
 		err := registry.Register(p.collector)
 		if err != nil {
 			return err

plugins/outputs/prometheus_client/prometheus_client_v2_test.go

@@ -48,6 +48,34 @@ func TestMetricVersion2(t *testing.T) {
 # HELP cpu_time_idle Telegraf collected metric
 # TYPE cpu_time_idle untyped
 cpu_time_idle{host="example.org"} 42
+`),
+		},
+		{
+			name: "when export timestamp is true timestamp is present in the metric",
+			output: &PrometheusClient{
+				Listen:            ":0",
+				MetricVersion:     2,
+				CollectorsExclude: []string{"gocollector", "process"},
+				Path:              "/metrics",
+				ExportTimestamp:   true,
+				Log:               Logger,
+			},
+			metrics: []telegraf.Metric{
+				testutil.MustMetric(
+					"cpu",
+					map[string]string{
+						"host": "example.org",
+					},
+					map[string]interface{}{
+						"time_idle": 42.0,
+					},
+					time.Unix(0, 0),
+				),
+			},
+			expected: []byte(`
+# HELP cpu_time_idle Telegraf collected metric
+# TYPE cpu_time_idle untyped
+cpu_time_idle{host="example.org"} 42 0
 `),
 		},
 		{

plugins/outputs/prometheus_client/v2/collector.go

@@ -43,11 +43,16 @@ type Collector struct {
 	coll           *serializer.Collection
 }
 
-func NewCollector(expire time.Duration, stringsAsLabel bool) *Collector {
+func NewCollector(expire time.Duration, stringsAsLabel bool, exportTimestamp bool) *Collector {
 	config := serializer.FormatConfig{}
 	if stringsAsLabel {
 		config.StringHandling = serializer.StringAsLabel
 	}
+
+	if exportTimestamp {
+		config.TimestampExport = serializer.ExportTimestamp
+	}
+
 	return &Collector{
 		expireDuration: expire,
 		coll:           serializer.NewCollection(config),