Add mutual TLS support to prometheus_client output plugin

plugins/outputs/prometheus_client/README.md

@@ -35,6 +35,10 @@ This plugin starts a [Prometheus](https://prometheus.io/) Client, it exposes all
   ## If set, enable TLS with the given certificate.
   # tls_cert = "/etc/ssl/telegraf.crt"
   # tls_key = "/etc/ssl/telegraf.key"
+
+  ## Set one or more allowed client CA certificate file names to
+  ## enable mutually authenticated TLS connections
+  # tls_allowed_cacerts = ["/etc/telegraf/clientca.pem"]
 
   ## Export metric collection time.
   # export_timestamp = false

plugins/outputs/prometheus_client/prometheus_client.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/internal"
+	"github.com/influxdata/telegraf/internal/tls"
 	"github.com/influxdata/telegraf/plugins/outputs"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -56,8 +57,6 @@ type MetricFamily struct {
 
 type PrometheusClient struct {
 	Listen             string
-	TLSCert            string            `toml:"tls_cert"`
-	TLSKey             string            `toml:"tls_key"`
 	BasicUsername      string            `toml:"basic_username"`
 	BasicPassword      string            `toml:"basic_password"`
 	IPRange            []string          `toml:"ip_range"`
@@ -67,6 +66,8 @@ type PrometheusClient struct {
 	StringAsLabel      bool              `toml:"string_as_label"`
 	ExportTimestamp    bool              `toml:"export_timestamp"`
 
+	tls.ServerConfig
+
 	server *http.Server
 
 	sync.Mutex
@@ -105,6 +106,10 @@ var sampleConfig = `
   ## If set, enable TLS with the given certificate.
   # tls_cert = "/etc/ssl/telegraf.crt"
   # tls_key = "/etc/ssl/telegraf.key"
+
+  ## Set one or more allowed client CA certificate file names to
+  ## enable mutually authenticated TLS connections
+  # tls_allowed_cacerts = ["/etc/telegraf/clientca.pem"]
 
   ## Export metric collection time.
   # export_timestamp = false
@@ -184,15 +189,20 @@ func (p *PrometheusClient) Connect() error {
 	mux.Handle(p.Path, p.auth(promhttp.HandlerFor(
 		registry, promhttp.HandlerOpts{ErrorHandling: promhttp.ContinueOnError})))
 
+	tlsConfig, err := p.TLSConfig()
+	if err != nil {
+		return err
+	}
 	p.server = &http.Server{
-		Addr:    p.Listen,
-		Handler: mux,
+		Addr:      p.Listen,
+		Handler:   mux,
+		TLSConfig: tlsConfig,
 	}
 
 	go func() {
 		var err error
 		if p.TLSCert != "" && p.TLSKey != "" {
-			err = p.server.ListenAndServeTLS(p.TLSCert, p.TLSKey)
+			err = p.server.ListenAndServeTLS("", "")
 		} else {
 			err = p.server.ListenAndServe()
 		}

plugins/outputs/prometheus_client/prometheus_client_tls_test.go

@@ -0,0 +1,104 @@
+package prometheus_client_test
+
+import (
+	"crypto/tls"
+	"fmt"
+	"github.com/influxdata/telegraf/plugins/outputs/prometheus_client"
+	"github.com/influxdata/telegraf/testutil"
+	"github.com/influxdata/toml"
+	"github.com/stretchr/testify/require"
+	"net/http"
+	"testing"
+)
+
+var pki = testutil.NewPKI("../../../testutil/pki")
+
+var configWithTLS = fmt.Sprintf(`
+ listen = "127.0.0.1:9090"
+ tls_allowed_cacerts = ["%s"]
+ tls_cert = "%s"
+ tls_key = "%s"
+`, pki.TLSServerConfig().TLSAllowedCACerts[0], pki.TLSServerConfig().TLSCert, pki.TLSServerConfig().TLSKey)
+
+var configWithoutTLS = `
+  listen = "127.0.0.1:9090"
+`
+
+type PrometheusClientTestContext struct {
+	Output      *prometheus_client.PrometheusClient
+	Accumulator *testutil.Accumulator
+	Client      *http.Client
+}
+
+func TestWorksWithoutTLS(t *testing.T) {
+	tc := buildTestContext(t, []byte(configWithoutTLS))
+	err := tc.Output.Connect()
+	defer tc.Output.Close()
+
+	require.NoError(t, err)
+
+	response, err := tc.Client.Get("http://localhost:9090/metrics")
+	require.NoError(t, err)
+
+	require.NoError(t, err)
+	require.Equal(t, response.StatusCode, http.StatusOK)
+}
+
+func TestWorksWithTLS(t *testing.T) {
+	tc := buildTestContext(t, []byte(configWithTLS))
+	err := tc.Output.Connect()
+	defer tc.Output.Close()
+	require.NoError(t, err)
+
+	response, err := tc.Client.Get("https://localhost:9090/metrics")
+	require.NoError(t, err)
+
+	require.NoError(t, err)
+	require.Equal(t, response.StatusCode, http.StatusOK)
+
+	response, err = tc.Client.Get("http://localhost:9090/metrics")
+	require.Error(t, err)
+
+	tr := &http.Transport{
+		TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
+	}
+
+	client := &http.Client{Transport: tr}
+	response, err = client.Get("https://localhost:9090/metrics")
+
+	require.Error(t, err)
+}
+
+func buildTestContext(t *testing.T, config []byte) *PrometheusClientTestContext {
+	output := prometheus_client.NewClient()
+	err := toml.Unmarshal(config, output)
+	require.NoError(t, err)
+
+	var (
+		httpClient *http.Client
+	)
+
+	if len(output.TLSAllowedCACerts) != 0 {
+		httpClient = buildClientWithTLS(t, output)
+	} else {
+		httpClient = buildClientWithoutTLS()
+	}
+
+	return &PrometheusClientTestContext{
+		Output:      output,
+		Accumulator: &testutil.Accumulator{},
+		Client:      httpClient,
+	}
+}
+
+func buildClientWithoutTLS() *http.Client {
+	return &http.Client{}
+}
+
+func buildClientWithTLS(t *testing.T, output *prometheus_client.PrometheusClient) *http.Client {
+	tlsConfig, err := pki.TLSClientConfig().TLSConfig()
+	require.NoError(t, err)
+
+	transport := &http.Transport{TLSClientConfig: tlsConfig}
+	return &http.Client{Transport: transport}
+}