There is a json collector plugin. It parses a json in a specific structure and the outputs can be String, Int etc. However, if the json is not following that structure, it might be possible to use the stdout collector which uses regexp to filter data from stdout, or we could add a collection plugin.
Did you create the command line tool yourself? That is probably the most important thing to decide which coarse of action to take.
The collector plugins are located in fastr/resources/collectorplugins. Sadly the documentation is currently lacking, but I will try to update it soon.