Add topic_tag option to mqtt_consumer

plugins/inputs/mqtt_consumer/README.md

@@ -18,6 +18,10 @@ and creates metrics using one of the supported [input data formats][].
     "sensors/#",
   ]
 
+  ## The message topic will be stored in a tag specified by this value.  If set
+  ## to the empty string no topic tag will be created.
+  # topic_tag = "topic"
+
   ## QoS policy for messages
   ##   0 = at most once
   ##   1 = at least once

plugins/inputs/mqtt_consumer/mqtt_consumer.go

@@ -43,10 +43,11 @@ type Client interface {
 type ClientFactory func(o *mqtt.ClientOptions) Client
 
 type MQTTConsumer struct {
-	Servers                []string
-	Topics                 []string
-	Username               string
-	Password               string
+	Servers                []string          `toml:"servers"`
+	Topics                 []string          `toml:"topics"`
+	TopicTag               *string           `toml:"topic_tag"`
+	Username               string            `toml:"username"`
+	Password               string            `toml:"password"`
 	QoS                    int               `toml:"qos"`
 	ConnectionTimeout      internal.Duration `toml:"connection_timeout"`
 	MaxUndeliveredMessages int               `toml:"max_undelivered_messages"`
@@ -67,6 +68,7 @@ type MQTTConsumer struct {
 	state         ConnectionState
 	sem           semaphore
 	messages      map[telegraf.TrackingID]bool
+	topicTag      string
 
 	ctx    context.Context
 	cancel context.CancelFunc
@@ -84,6 +86,10 @@ var sampleConfig = `
     "sensors/#",
   ]
 
+  ## The message topic will be stored in a tag specified by this value.  If set
+  ## to the empty string no topic tag will be created.
+  # topic_tag = "topic"
+
   ## QoS policy for messages
   ##   0 = at most once
   ##   1 = at least once
@@ -161,6 +167,11 @@ func (m *MQTTConsumer) Init() error {
 		return fmt.Errorf("connection_timeout must be greater than 1s: %s", m.ConnectionTimeout.Duration)
 	}
 
+	m.topicTag = "topic"
+	if m.TopicTag != nil {
+		m.topicTag = *m.TopicTag
+	}
+
 	opts, err := m.createOpts()
 	if err != nil {
 		return err
@@ -267,9 +278,11 @@ func (m *MQTTConsumer) onMessage(acc telegraf.TrackingAccumulator, msg mqtt.Mess
 		return err
 	}
 
-	topic := msg.Topic()
-	for _, metric := range metrics {
-		metric.AddTag("topic", topic)
+	if m.topicTag != "" {
+		topic := msg.Topic()
+		for _, metric := range metrics {
+			metric.AddTag(m.topicTag, topic)
+		}
 	}
 
 	id := acc.AddTrackingMetricGroup(metrics)

plugins/inputs/mqtt_consumer/mqtt_consumer_test.go

@@ -143,6 +143,142 @@ func TestPersistentClientIDFail(t *testing.T) {
 	require.Error(t, err)
 }
 
+type Message struct {
+}
+
+func (m *Message) Duplicate() bool {
+	panic("not implemented")
+}
+
+func (m *Message) Qos() byte {
+	panic("not implemented")
+}
+
+func (m *Message) Retained() bool {
+	panic("not implemented")
+}
+
+func (m *Message) Topic() string {
+	return "telegraf"
+}
+
+func (m *Message) MessageID() uint16 {
+	panic("not implemented")
+}
+
+func (m *Message) Payload() []byte {
+	return []byte("cpu time_idle=42i")
+}
+
+func (m *Message) Ack() {
+	panic("not implemented")
+}
+
+func TestTopicTag(t *testing.T) {
+	tests := []struct {
+		name     string
+		topicTag func() *string
+		expected []telegraf.Metric
+	}{
+		{
+			name: "default topic when topic tag is unset for backwards compatibility",
+			topicTag: func() *string {
+				return nil
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric(
+					"cpu",
+					map[string]string{
+						"topic": "telegraf",
+					},
+					map[string]interface{}{
+						"time_idle": 42,
+					},
+					time.Unix(0, 0),
+				),
+			},
+		},
+		{
+			name: "use topic tag when set",
+			topicTag: func() *string {
+				tag := "topic_tag"
+				return &tag
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric(
+					"cpu",
+					map[string]string{
+						"topic_tag": "telegraf",
+					},
+					map[string]interface{}{
+						"time_idle": 42,
+					},
+					time.Unix(0, 0),
+				),
+			},
+		},
+		{
+			name: "no topic tag is added when topic tag is set to the empty string",
+			topicTag: func() *string {
+				tag := ""
+				return &tag
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric(
+					"cpu",
+					map[string]string{},
+					map[string]interface{}{
+						"time_idle": 42,
+					},
+					time.Unix(0, 0),
+				),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var handler mqtt.MessageHandler
+			client := &FakeClient{
+				ConnectF: func() mqtt.Token {
+					return &FakeToken{}
+				},
+				AddRouteF: func(topic string, callback mqtt.MessageHandler) {
+					handler = callback
+				},
+				SubscribeMultipleF: func(filters map[string]byte, callback mqtt.MessageHandler) mqtt.Token {
+					return &FakeToken{}
+				},
+				DisconnectF: func(quiesce uint) {
+				},
+			}
+
+			plugin := New(func(o *mqtt.ClientOptions) Client {
+				return client
+			})
+			plugin.Topics = []string{"telegraf"}
+			plugin.TopicTag = tt.topicTag()
+
+			parser, err := parsers.NewInfluxParser()
+			require.NoError(t, err)
+			plugin.SetParser(parser)
+
+			err = plugin.Init()
+			require.NoError(t, err)
+
+			var acc testutil.Accumulator
+			err = plugin.Start(&acc)
+			require.NoError(t, err)
+
+			handler(nil, &Message{})
+
+			plugin.Stop()
+
+			testutil.RequireMetricsEqual(t, tt.expected, acc.GetTelegrafMetrics(),
+				testutil.IgnoreTime())
+		})
+	}
+}
+
 func TestAddRouteCalledForEachTopic(t *testing.T) {
 	client := &FakeClient{
 		ConnectF: func() mqtt.Token {