Convert bool fields to int in graphite serializer

docs/DATA_FORMATS_OUTPUT.md

@@ -96,7 +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.
+Fields with string values will be skipped.  Boolean fields will be converted
+to 1 (true) or 0 (false).
 
 ### Graphite Configuration:
 

plugins/serializers/graphite/graphite.go

@@ -32,9 +32,15 @@ func (s *GraphiteSerializer) Serialize(metric telegraf.Metric) ([]byte, error) {
 	}
 
 	for fieldName, value := range metric.Fields() {
-		switch value.(type) {
+		switch v := value.(type) {
 		case string:
 			continue
+		case bool:
+			if v {
+				value = 1
+			} else {
+				value = 0
+			}
 		}
 		metricString := fmt.Sprintf("%s %#v %d\n",
 			// insert "field" section of template

plugins/serializers/graphite/graphite_test.go

@@ -187,6 +187,34 @@ func TestSerializeValueString(t *testing.T) {
 	assert.Equal(t, "", mS[0])
 }
 
+func TestSerializeValueBoolean(t *testing.T) {
+	now := time.Now()
+	tags := map[string]string{
+		"host":       "localhost",
+		"cpu":        "cpu0",
+		"datacenter": "us-west-2",
+	}
+	fields := map[string]interface{}{
+		"enabled":  true,
+		"disabled": false,
+	}
+	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)
+
+	expS := []string{
+		fmt.Sprintf("localhost.enabled.cpu0.us-west-2.cpu 1 %d", now.Unix()),
+		fmt.Sprintf("localhost.disabled.cpu0.us-west-2.cpu 0 %d", now.Unix()),
+	}
+	assert.Equal(t, expS, mS)
+}
+
 // test that fields with spaces get fixed.
 func TestSerializeFieldWithSpaces(t *testing.T) {
 	now := time.Now()