docs(inputs.prometheus): Add example for prometheus timestamps (#13362)

plugins/inputs/prometheus/README.md

@@ -402,3 +402,31 @@ prometheus,cpu=cpu1,url=http://example.org:9273/metrics cpu_usage_user=5.8291457
 prometheus,cpu=cpu2,url=http://example.org:9273/metrics cpu_usage_user=2.119071644805144 1505776751000000000
 prometheus,cpu=cpu3,url=http://example.org:9273/metrics cpu_usage_user=1.5228426395944945 1505776751000000000
 ```
+
+### Output with timestamp included
+
+Below is an example of a Prometheus metric which includes a timestamp:
+
+```text
+# TYPE test_counter counter
+test_counter{label="test"} 1 1685443805885
+```
+
+Telegraf will generate the following metric:
+
+```text
+test_counter,address=127.0.0.1,label=test counter=1 1685443805885000000
+```
+
+using the standard configuration
+
+```toml
+[[inputs.prometheus]]
+  ## An array of urls to scrape metrics from.
+  urls = ["http://localhost:2019/metrics"]
+```
+
+**Please note:** Metrics generated by Prometheus endpoints are generated with
+*millisecond precision*. The default Telegraf agent level precision setting
+reduces this to seconds. Change the `precision` setting at agent or plugin level
+to milliseconds or smaller to report metric timestamps with full precision.

plugins/inputs/prometheus/prometheus_test.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf/config"
+	"github.com/influxdata/telegraf/metric"
 
 	"github.com/stretchr/testify/require"
 	"k8s.io/apimachinery/pkg/fields"
@@ -168,6 +169,41 @@ func TestPrometheusGeneratesMetricsWithHostNameTag(t *testing.T) {
 	require.True(t, acc.TagValue("test_metric", "url") == ts.URL)
 }
 
+func TestPrometheusWithTimestamp(t *testing.T) {
+	prommetric := `# HELP test_counter A sample test counter.
+# TYPE test_counter counter
+test_counter{label="test"} 1 1685443805885`
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, err := fmt.Fprintln(w, prommetric)
+		require.NoError(t, err)
+	}))
+	defer ts.Close()
+
+	p := &Prometheus{
+		Log:                testutil.Logger{},
+		KubernetesServices: []string{ts.URL},
+	}
+	require.NoError(t, p.Init())
+
+	u, err := url.Parse(ts.URL)
+	require.NoError(t, err)
+	tsAddress := u.Hostname()
+
+	expected := []telegraf.Metric{
+		metric.New(
+			"test_counter",
+			map[string]string{"address": tsAddress, "label": "test"},
+			map[string]interface{}{"counter": float64(1.0)},
+			time.UnixMilli(1685443805885),
+			telegraf.Counter,
+		),
+	}
+
+	var acc testutil.Accumulator
+	require.NoError(t, acc.GatherError(p.Gather))
+	testutil.RequireMetricsEqual(t, expected, acc.GetTelegrafMetrics())
+}
+
 func TestPrometheusGeneratesMetricsAlthoughFirstDNSFailsIntegration(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping integration test in short mode")