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

plugins/outputs/mqtt/README.md

@@ -32,10 +32,14 @@ This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt
   # tls_key = "/etc/telegraf/key.pem"
   ## Use TLS but skip chain & host verification
   # insecure_skip_verify = false
-
+  
   ## When true, metrics will be sent in one MQTT message per flush.  Otherwise,
   ## 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)