Add option to set retain flag on messages in mqtt output (#4892)

plugins/outputs/mqtt/README.md

@@ -37,6 +37,10 @@ This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt
   ## metrics are written one metric per MQTT message.
   # batch = false
   
+  ## When true, metric will have RETAIN flag set, making broker cache entries until someone
+  ## actually reads it
+  # retain = flase
+
   ## Data format to output.
   # data_format = "influx"
 ```
@@ -56,4 +60,5 @@ This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt
 * `tls_cert`: TLS CERT
 * `tls_key`: TLS key
 * `insecure_skip_verify`: Use TLS but skip chain & host verification (default: false)
+* `retain`: Set `retain` flag when publishing, instructing server to cache metric until someone reads it (default: false)
 * `data_format`: [About Telegraf data formats](https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md)

plugins/outputs/mqtt/mqtt.go

@@ -50,6 +50,10 @@ var sampleConfig = `
   ## metrics are written one metric per MQTT message.
   # batch = false
   
+  ## When true, metric will have RETAIN flag set, making broker cache entries until someone
+  ## actually reads it
+  # retain = flase
+
   ## Data format to output.
   ## Each data format has its own unique set of configuration options, read
   ## more about them here:
@@ -68,6 +72,7 @@ type MQTT struct {
 	ClientID    string `toml:"client_id"`
 	tls.ClientConfig
 	BatchMessage bool `toml:"batch"`
+	Retain       bool
 
 	client paho.Client
 	opts   *paho.ClientOptions
@@ -174,7 +179,7 @@ func (m *MQTT) Write(metrics []telegraf.Metric) error {
 }
 
 func (m *MQTT) publish(topic string, body []byte) error {
-	token := m.client.Publish(topic, byte(m.QoS), false, body)
+	token := m.client.Publish(topic, byte(m.QoS), m.Retain, body)
 	token.WaitTimeout(m.Timeout.Duration)
 	if token.Error() != nil {
 		return token.Error()