Wavefront data format rejects delta metrics

[snltd]

Relevant telegraf.conf:

[[inputs.socket_listener]]
  service_address = "unix:///tmp/telegraf.sock"
  data_format = "wavefront"

System info:

Telegraf 1.9.0

Steps to reproduce:

1. Start telegraf with above config
2. Send a delta metric via the Unix domain socket. Using the Wavefront CLI:

$ wf write point -i -u unix -S /tmp/telegraf.sock example.delta 1

Expected behavior:

Delta metric is forwarded to Wavefront proxy and thence to Wavefront.

Actual behavior:

Metric is not forwarded to proxy: this message in telegraf.log

2018-12-05T18:09:36Z E! [inputs.socket_listener]: Error in plugin: unable to parse incoming line: invalid metric value

[glinton]

Can you retry with the metric name in double quotes as their docs suggest?
wf write point -i -u unix -S /tmp/telegraf.sock "example.delta" 1