Add optional field to json

[gs0510]

Would it be possible to add an optional field in the JSON object that current-bench expects, e.g. each result item would have the fields {name; metrics; output} (or log), the contents of this last field would just be printed verbatim in the log.

From #285, it would be nice if we can provide an optional field in the json, where the users can see things in the log that they want to see, we won't have to process anything, just print the logs verbatim.

[gs0510]

cc @art-w @punchagan What are your thoughts? :)

[punchagan]

This would be doable, I think. But, I'd like to understand why this feature was requested, a little better. Would it make the logs more readable? What kind of logs/outputs do we see people adding in this field?