feat(outputs.loki): add option for metric name label

This allows the user to specify the label name used to store the metric
name. This label was added in influxdata#10001, but a user may not want this value
in which case they could set it to an empty string or use some other
customer value.

fixes: influxdata#13146

plugins/outputs/loki/README.md

@@ -52,4 +52,10 @@ to use them.
   # tls_ca = "/etc/telegraf/ca.pem"
   # tls_cert = "/etc/telegraf/cert.pem"
   # tls_key = "/etc/telegraf/key.pem"
+
+  ## Metric Name Label
+  ## Label to use for the metric name to when sending metrics. If set to an
+  ## empty string, this will not add the label. This is NOT suggested as there
+  ## is no way to differentiate between multiple metrics.
+  # metric_name_label = "__name"
 ```

plugins/outputs/loki/loki.go

@@ -32,17 +32,18 @@ const (
 )
 
 type Loki struct {
-	Domain       string            `toml:"domain"`
-	Endpoint     string            `toml:"endpoint"`
-	Timeout      config.Duration   `toml:"timeout"`
-	Username     config.Secret     `toml:"username"`
-	Password     config.Secret     `toml:"password"`
-	Headers      map[string]string `toml:"http_headers"`
-	ClientID     string            `toml:"client_id"`
-	ClientSecret string            `toml:"client_secret"`
-	TokenURL     string            `toml:"token_url"`
-	Scopes       []string          `toml:"scopes"`
-	GZipRequest  bool              `toml:"gzip_request"`
+	Domain          string            `toml:"domain"`
+	Endpoint        string            `toml:"endpoint"`
+	Timeout         config.Duration   `toml:"timeout"`
+	Username        config.Secret     `toml:"username"`
+	Password        config.Secret     `toml:"password"`
+	Headers         map[string]string `toml:"http_headers"`
+	ClientID        string            `toml:"client_id"`
+	ClientSecret    string            `toml:"client_secret"`
+	TokenURL        string            `toml:"token_url"`
+	Scopes          []string          `toml:"scopes"`
+	GZipRequest     bool              `toml:"gzip_request"`
+	MetricNameLabel *string           `toml:"metric_name_label"`
 
 	url    string
 	client *http.Client
@@ -77,6 +78,15 @@ func (l *Loki) createClient(ctx context.Context) (*http.Client, error) {
 	return client, nil
 }
 
+func (l *Loki) Init() error {
+	if l.MetricNameLabel == nil {
+		defaultMetricName := "__name"
+		l.MetricNameLabel = &defaultMetricName
+	}
+
+	return nil
+}
+
 func (*Loki) SampleConfig() string {
 	return sampleConfig
 }
@@ -119,7 +129,9 @@ func (l *Loki) Write(metrics []telegraf.Metric) error {
 	})
 
 	for _, m := range metrics {
-		m.AddTag("__name", m.Name())
+		if *l.MetricNameLabel != "" {
+			m.AddTag(*l.MetricNameLabel, m.Name())
+		}
 
 		tags := m.TagList()
 		var line string

plugins/outputs/loki/loki_test.go

@@ -243,6 +243,68 @@ func TestContentEncodingGzip(t *testing.T) {
 	}
 }
 
+func TestMetricNameLabel(t *testing.T) {
+	ts := httptest.NewServer(http.NotFoundHandler())
+	defer ts.Close()
+
+	u, err := url.Parse(fmt.Sprintf("http://%s", ts.Listener.Addr().String()))
+	require.NoError(t, err)
+
+	tests := []struct {
+		name            string
+		metricNameLabel string
+	}{
+		{
+			name:            "ensure default value of __name",
+			metricNameLabel: "nil",
+		},
+		{
+			name:            "no label",
+			metricNameLabel: "",
+		},
+		{
+			name:            "custom label",
+			metricNameLabel: "foobar",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ts.Config.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				payload, err := io.ReadAll(r.Body)
+				require.NoError(t, err)
+
+				var s Request
+				require.NoError(t, json.Unmarshal(payload, &s))
+
+				switch tt.metricNameLabel {
+				case "nil":
+					require.Equal(t, map[string]string{"__name": "log", "key1": "value1"}, s.Streams[0].Labels)
+				case "":
+					require.Equal(t, map[string]string{"key1": "value1"}, s.Streams[0].Labels)
+				case "foobar":
+					require.Equal(t, map[string]string{"foobar": "log", "key1": "value1"}, s.Streams[0].Labels)
+				}
+
+				w.WriteHeader(http.StatusNoContent)
+			})
+
+			var metricNameLabel *string
+			if tt.metricNameLabel != "nil" {
+				metricNameLabel = &tt.metricNameLabel //nolint:revive // different values
+			}
+
+			l := Loki{
+				Domain:          u.String(),
+				MetricNameLabel: metricNameLabel,
+			}
+			require.NoError(t, l.Init())
+			require.NoError(t, l.Connect())
+			require.NoError(t, l.Write([]telegraf.Metric{getMetric()}))
+		})
+	}
+}
+
 func TestBasicAuth(t *testing.T) {
 	ts := httptest.NewServer(http.NotFoundHandler())
 	defer ts.Close()

plugins/outputs/loki/sample.conf

@@ -23,3 +23,9 @@
   # tls_ca = "/etc/telegraf/ca.pem"
   # tls_cert = "/etc/telegraf/cert.pem"
   # tls_key = "/etc/telegraf/key.pem"
+
+  ## Metric Name Label
+  ## Label to use for the metric name to when sending metrics. If set to an
+  ## empty string, this will not add the label. This is NOT suggested as there
+  ## is no way to differentiate between multiple metrics.
+  # metric_name_label = "__name"