metrics diff: support numeric string delta for json metrics

[DavidGOrtega]

Hi, python is very problematic serialising JSON floats. With the standard library is very easy to incur in

TypeError: 0.873 is not JSON serializable

also if you specify the value as a float you are giving the control to the JSON parser which may not be very precise, and precision may be here very important.

a_ver :

{"accuracy": "0.8729"}

b_ver

{"accuracy":  "0.8755"}

outputs no diff

{"metrics/eval.json": {"accuracy": {"old": "0.8729", "new": "0.8755"}}} 

or

      Path           Metric    Value         Change                     
metrics/eval.json   accuracy   0.875   diff not supported

[efiop]

@DavidGOrtega Could you please show where TypeError: 0.873 is not JSON serializable came from?

[DavidGOrtega]

@efiop what I say is that if you do json.dump( of floats in my metrics with python I have that error, thats why I store them as strings. Said that also is a good way to store them as strings if you want to be very precise because if not you rely on the json decoder.

[efiop]

@DavidGOrtega I just don't quite understand how to reproduce your issue:

>>> import json
>>> json.dumps({"a": 0.873})
'{"a": 0.873}'           

works fine for me.

[DavidGOrtega]

Its tricky but very well known:
https://bugs.python.org/issue16535

Aside of that I like to store in string not handle my self the precision

[efiop]

@DavidGOrtega To my shame, I am not very familiar with that bug. Seems like it is only related to Decimal not being JSON serializable, not sure it is worth adding the magic for. CC @dmpetrov @iterative/engineering

[efiop]

Closing for now.