InfluxDB UDP output plugin not splitting large measurements

[oplehto]

Telegraf's UDP output is not splitting outgoing measurement lines based on the udp_payload. It seems like the this block is never executed as serializer alway exists: https://github.com/influxdata/telegraf/blob/master/plugins/outputs/influxdb/udp.go#L46

Relevant telegraf.conf:

[[outputs.influxdb]]
  urls = ['udp://metrics-udp:22001/']
  database = "tg_udp"
  precision = "s"
  timeout = "5s"
  user_agent = "telegraf"
  udp_payload = 1400

System info:

Tested on Telegraf 1.9.4 but issue seems to be still in HEAD

Steps to reproduce:

1. Set a udp_payload value in the [[outputs.influxdb]] plugin which is lower the size of some measurement lines.
2. Check the UDP datagram sizes with tcpdump.

Expected behavior:

The datagrams should be split to be under the udp_payload size.

Actual behavior:

The udp_payload parameter is disregarded and no split is performed. Measurements larger than udp_payload size are sent as single datagrams. For very large measurements a "message too long" error is produced.