Consider adding richer episode stats in JSON (BLOCKED: do this after supporting more types of log file)

[dmh23]

As follow up to #186 which focussed on training parameters, later on we should do similar work on the JSON stats - but ideally do this once Guru supports additional log types (see #142), since the proper JSON data structure will depend somewhat on the range of type of useful attributes for those other log types, and the overlap with existing stats for training logs.