How can I use FuncMap to help transform data from the log, inside a template?

[mad-ady]

Hello - I'm parsing a log that has a HEX representation of an ASCII string somewhere in the line. For instance: thing=313241414242, which decoded results in thing=12AABB. I'd like to parse this string and report the decoded ASCII string back to prometheus as a label.

The grok configuration looks something like this:

- type: counter
          name: thing_counter
          help: Test thing counter
          match: 'thing=(?<thing_hex>[0-9A-F]+)'
          retention: 5m
          path: 'thing.log'
          labels:
            host: 'thing_host'
            thing: "{{- printf \"%s\"  (hex.DecodeString .thing_hex)  -}}"

I'm trying to use the DecodeString function from the hex package. However, when I try to start grok_exporter, it complains that:

Failed to load ...: invalid configuration: failed to read metric label thing_counter: error parsing thing template: template: thing:1: function "hex" not defined: don't forget to put a . (dot) in front of grok fields, otherwise it will be interpreted as a function.

I'm not skilled in go, nor its templating engine, but I learned I have to use FuncMap to pass the names of the functions that I need to the template. Any suggestions on how I can use FuncMap from within grok_exporter to be able to access hex.DecodeString?

Thanks