Support Landing page on Prometheus landing page (#8641)

plugins/outputs/prometheus_client/prometheus_client.go

@@ -159,12 +159,16 @@ func (p *PrometheusClient) Init() error {
 	authHandler := internal.AuthHandler(p.BasicUsername, p.BasicPassword, "prometheus", onAuthError)
 	rangeHandler := internal.IPRangeHandler(ipRange, onError)
 	promHandler := promhttp.HandlerFor(registry, promhttp.HandlerOpts{ErrorHandling: promhttp.ContinueOnError})
+	landingPageHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("Telegraf Output Plugin: Prometheus Client "))
+	})
 
 	mux := http.NewServeMux()
 	if p.Path == "" {
-		p.Path = "/"
+		p.Path = "/metrics"
 	}
 	mux.Handle(p.Path, authHandler(rangeHandler(promHandler)))
+	mux.Handle("/", authHandler(rangeHandler(landingPageHandler)))
 
 	tlsConfig, err := p.TLSConfig()
 	if err != nil {

plugins/outputs/prometheus_client/prometheus_client_v1_test.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"strings"
 	"testing"
 	"time"
@@ -400,3 +401,29 @@ rpc_duration_seconds_count 2693
 		})
 	}
 }
+
+func TestLandingPage(t *testing.T) {
+	Logger := testutil.Logger{Name: "outputs.prometheus_client"}
+	output := PrometheusClient{
+		Listen:            ":0",
+		CollectorsExclude: []string{"process"},
+		MetricVersion:     1,
+		Log:               Logger,
+	}
+	expected := "Telegraf Output Plugin: Prometheus Client"
+
+	err := output.Init()
+	require.NoError(t, err)
+
+	err = output.Connect()
+	require.NoError(t, err)
+
+	u, err := url.Parse(fmt.Sprintf("http://%s/", output.url.Host))
+	resp, err := http.Get(u.String())
+	require.NoError(t, err)
+
+	actual, err := ioutil.ReadAll(resp.Body)
+	require.NoError(t, err)
+
+	require.Equal(t, expected, strings.TrimSpace(string(actual)))
+}