BUG/MINOR: stats: fix validity of the json schema

BUG/MINOR: stats: fix validity of the json schema

The json schema seems to be invalid when checking using the validator
from https://www.jsonschemavalidator.net/. Correct it using the
following specification :
http://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.9.1

The impact of the bug it not well known as I am not sure of how useful
the json schema is for users. It is probably not used at all or else
this bug would have been reported.

This should be backported up to 1.8.