_timestamp field in mqtt message causes logstash ingestion failure

[tedder]

I have some mqtt messages coming in that already have a _timestamp field, and logstash complains about these.

2018/09/27 17:38:42.251161 mqttbeat.go:87: DBG MQTT MESSAGE RECEIVED {"_timestamp": 1538069922, "actual": 21.6, "target": 0.0}
2018/09/27 17:38:42.251298 mqttbeat.go:147: DBG Payload decoded - json
2018/09/27 17:38:42.251421 client.go:203: DBG Publish: {
  "@timestamp": "2018-09-27T17:38:42.251Z",
  "_timestamp": 1538069922,
  "actual": 21.6,
  "beat": {
    "index": "mqttbeat",
    "type": "message"

I don't know what the answer is since I can't change my incoming messages, but I at least wanted to log it.