Skip non-numerical values in graphite format (#3179)

docs/DATA_FORMATS_OUTPUT.md

@@ -96,6 +96,8 @@ tars.cpu-total.us-east-1.cpu.usage_user 0.89 1455320690
 tars.cpu-total.us-east-1.cpu.usage_idle 98.09 1455320690
 ```
 
+Fields with non-numeric values will be skipped.
+
 ### Graphite Configuration:
 
 ```toml

plugins/serializers/graphite/graphite.go

@@ -32,13 +32,16 @@ func (s *GraphiteSerializer) Serialize(metric telegraf.Metric) ([]byte, error) {
 	}
 
 	for fieldName, value := range metric.Fields() {
-		// Convert value to string
-		valueS := fmt.Sprintf("%#v", value)
-		point := []byte(fmt.Sprintf("%s %s %d\n",
+		switch value.(type) {
+		case string:
+			continue
+		}
+		metricString := fmt.Sprintf("%s %#v %d\n",
 			// insert "field" section of template
 			sanitizedChars.Replace(InsertField(bucket, fieldName)),
-			sanitizedChars.Replace(valueS),
-			timestamp))
+			value,
+			timestamp)
+		point := []byte(metricString)
 		out = append(out, point...)
 	}
 	return out, nil

plugins/serializers/graphite/graphite_test.go

@@ -165,6 +165,28 @@ func TestSerializeValueField2(t *testing.T) {
 	assert.Equal(t, expS, mS)
 }
 
+func TestSerializeValueString(t *testing.T) {
+	now := time.Now()
+	tags := map[string]string{
+		"host":       "localhost",
+		"cpu":        "cpu0",
+		"datacenter": "us-west-2",
+	}
+	fields := map[string]interface{}{
+		"value": "asdasd",
+	}
+	m, err := metric.New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+
+	s := GraphiteSerializer{
+		Template: "host.field.tags.measurement",
+	}
+	buf, _ := s.Serialize(m)
+	mS := strings.Split(strings.TrimSpace(string(buf)), "\n")
+	assert.NoError(t, err)
+	assert.Equal(t, "", mS[0])
+}
+
 // test that fields with spaces get fixed.
 func TestSerializeFieldWithSpaces(t *testing.T) {
 	now := time.Now()