Add option to disable labels in prometheus output for string fields (#3765)

plugins/outputs/prometheus_client/README.md

@@ -23,4 +23,8 @@ This plugin starts a [Prometheus](https://prometheus.io/) Client, it exposes all
 
   # Expiration interval for each metric. 0 == no expiration
   expiration_interval = "60s"
+
+  # Send string metrics as Prometheus labels.
+  # Unless set to false all string metrics will be sent as labels.
+  string_as_label = true
 ```

plugins/outputs/prometheus_client/prometheus_client.go

@@ -61,6 +61,7 @@ type PrometheusClient struct {
 	ExpirationInterval internal.Duration `toml:"expiration_interval"`
 	Path               string            `toml:"path"`
 	CollectorsExclude  []string          `toml:"collectors_exclude"`
+	StringAsLabel      bool              `toml:"string_as_label"`
 
 	server *http.Server
 
@@ -89,6 +90,10 @@ var sampleConfig = `
   ## Collectors to enable, valid entries are "gocollector" and "process".
   ## If unset, both are enabled.
   collectors_exclude = ["gocollector", "process"]
+
+  # Send string metrics as Prometheus labels.
+  # Unless set to false all string metrics will be sent as labels.
+  string_as_label = true
 `
 
 func (p *PrometheusClient) basicAuth(h http.Handler) http.Handler {
@@ -326,11 +331,13 @@ func (p *PrometheusClient) Write(metrics []telegraf.Metric) error {
 		}
 
 		// Prometheus doesn't have a string value type, so convert string
-		// fields to labels.
-		for fn, fv := range point.Fields() {
-			switch fv := fv.(type) {
-			case string:
-				labels[sanitize(fn)] = fv
+		// fields to labels if enabled.
+		if p.StringAsLabel {
+			for fn, fv := range point.Fields() {
+				switch fv := fv.(type) {
+				case string:
+					labels[sanitize(fn)] = fv
+				}
 			}
 		}
 
@@ -465,6 +472,7 @@ func init() {
 	outputs.Add("prometheus_client", func() telegraf.Output {
 		return &PrometheusClient{
 			ExpirationInterval: internal.Duration{Duration: time.Second * 60},
+			StringAsLabel:      true,
 			fam:                make(map[string]*MetricFamily),
 			now:                time.Now,
 		}

plugins/outputs/prometheus_client/prometheus_client_test.go

@@ -22,6 +22,7 @@ func setUnixTime(client *PrometheusClient, sec int64) {
 func NewClient() *PrometheusClient {
 	return &PrometheusClient{
 		ExpirationInterval: internal.Duration{Duration: time.Second * 60},
+		StringAsLabel:      true,
 		fam:                make(map[string]*MetricFamily),
 		now:                time.Now,
 	}
@@ -450,6 +451,40 @@ func TestWrite_StringFields(t *testing.T) {
 	require.False(t, ok)
 }
 
+func TestDoNotWrite_StringFields(t *testing.T) {
+	now := time.Now()
+	p1, err := metric.New(
+		"foo",
+		make(map[string]string),
+		map[string]interface{}{"value": 1.0, "status": "good"},
+		now,
+		telegraf.Counter)
+	p2, err := metric.New(
+		"bar",
+		make(map[string]string),
+		map[string]interface{}{"status": "needs numeric field"},
+		now,
+		telegraf.Gauge)
+	var metrics = []telegraf.Metric{p1, p2}
+
+	client := &PrometheusClient{
+		ExpirationInterval: internal.Duration{Duration: time.Second * 60},
+		StringAsLabel:      false,
+		fam:                make(map[string]*MetricFamily),
+		now:                time.Now,
+	}
+
+	err = client.Write(metrics)
+	require.NoError(t, err)
+
+	fam, ok := client.fam["foo"]
+	require.True(t, ok)
+	require.Equal(t, 0, fam.LabelSet["status"])
+
+	fam, ok = client.fam["bar"]
+	require.False(t, ok)
+}
+
 func TestExpire(t *testing.T) {
 	client := NewClient()