Output exec plugin does not use Serialise batch function

[morfien101]

Relevant telegraf.conf:

[[output.exec]]
  command = ["cat"]
  data_format = "json"

System info:

Any system
Telegraf 1.12.2 (git: HEAD 8b4c9a0)

Steps to reproduce:

Send more than a single metric into the output stream

Expected behavior:

Data format should be parsable using the format specified.

Actual behavior:

Data is given as if it was a single metric over many lines.