jsonnet $lookup function breaks the config in 1.27

[khash]

Relevant telegraf.conf

[[outputs.http]]
  ...
  json_transformation = "$lookup($, 'metrics') ? metrics.{'name': $.name,'host': $.tags.host,'fields': $.fields,'tags': $.tags,'timestamp': $.timestamp} : [{'name': $.name,'host': $.tags.host,'fields': $.fields,'tags': $.tags,'timestamp': $.timestamp}]"

Logs from Telegraf

WARN[0000] The "lookup" variable is not set. Defaulting to a blank string.

System info

Ubuntu 20.04

Docker

No response

Steps to reproduce

1. Use $lookup in a JSON transformer that works in 1.26.3 and then upgrade to 1.27.0 you will see the following warning and the transformer stops working: WARN[0000] The "lookup" variable is not set. Defaulting to a blank string.
2. Replace $lookup with $$lookup in the config and 1.27.0 will work, but it won't work with 1.26.3

Expected behavior

Config from version 1.26.3 to work with 1.27.0 when containing jsonnet $ functions, like $lookup

Actual behavior

It should be possible to continue to use jsonnet $ functions in JSON transformer configs.

Additional info

The latest release, throws the following warning if $lookup (a jsonnet function) is used as a JSON transformer:

WARN[0000] The "lookup" variable is not set. Defaulting to a blank string.

Looking at the changelog notes, it might be due to the replacement of the parser. Given that this error is a generic Golang parser error, replacing the $lookup with $$lookup fixes the issue, but this means potentially many broken json transformers as this release is being used.

[powersj]

Hi,

Closing this as a dup of #13432, working through the fix. Sorry about the regression!