fix(outputs.prometheus_client): Fix export_timestamp for v1 metric type (#13169)

Co-authored-by: Roman Orlov <orlov@farpost.com>

plugins/outputs/prometheus_client/prometheus_client.go

@@ -100,7 +100,7 @@ func (p *PrometheusClient) Init() error {
 	default:
 		fallthrough
 	case 1:
-		p.collector = v1.NewCollector(time.Duration(p.ExpirationInterval), p.StringAsLabel, p.Log)
+		p.collector = v1.NewCollector(time.Duration(p.ExpirationInterval), p.StringAsLabel, p.ExportTimestamp, p.Log)
 		err := registry.Register(p.collector)
 		if err != nil {
 			return err

plugins/outputs/prometheus_client/prometheus_client_v1_test.go

@@ -105,6 +105,35 @@ cpu_time_idle{host="example.org"} 42
 # HELP cpu_time_idle Telegraf collected metric
 # TYPE cpu_time_idle counter
 cpu_time_idle{host="example.org"} 42
+`),
+		},
+		{
+			name: "when export timestamp is true timestamp is present in the metric",
+			output: &PrometheusClient{
+				Listen:            ":0",
+				MetricVersion:     1,
+				CollectorsExclude: []string{"gocollector", "process"},
+				Path:              "/metrics",
+				ExportTimestamp:   true,
+				Log:               logger,
+			},
+			metrics: []telegraf.Metric{
+				testutil.MustMetric(
+					"cpu_time_idle",
+					map[string]string{
+						"host": "example.org",
+					},
+					map[string]interface{}{
+						"counter": 42.0,
+					},
+					time.Unix(1257894000, 0),
+					telegraf.Counter,
+				),
+			},
+			expected: []byte(`
+# HELP cpu_time_idle Telegraf collected metric
+# TYPE cpu_time_idle counter
+cpu_time_idle{host="example.org"} 42 1257894000000
 `),
 		},
 		{

plugins/outputs/prometheus_client/v1/collector.go

@@ -61,10 +61,11 @@ type Collector struct {
 	expireTicker *time.Ticker
 }
 
-func NewCollector(expire time.Duration, stringsAsLabel bool, logger telegraf.Logger) *Collector {
+func NewCollector(expire time.Duration, stringsAsLabel bool, exportTimestamp bool, logger telegraf.Logger) *Collector {
 	c := &Collector{
 		ExpirationInterval: expire,
 		StringAsLabel:      stringsAsLabel,
+		ExportTimestamp:    exportTimestamp,
 		Log:                logger,
 		fam:                make(map[string]*MetricFamily),
 	}