anchor.diff:  <1> The fields to compute and return field stats for.
   <2> The set index constraints. Note that index constrains can be defined for fields that aren't defined in the `fields` option.
   <3> Index constraints for the field `creation_date`.
  -<4> An index constraint on the `min_value` property of a field statistic.
  +<4> Index constraints on the `max_value` and `min_value` property of a field statistic.
   
   For a field, index constraints can be defined on the `min_value` statistic, `max_value` statistic or both.
   Each index constraint support the following comparisons:
  
positive.code:    "fields" : ["answer_count"]
     "index_constraints" : {
        "creation_date" : {
           "min_value" : {
              "gte" : "2014-01-01",
              "format" : "date_optional_time" <1>
           },
           "max_value" : {
              "lt" : "2015-01-01",
              "format" : "date_optional_time"
           }
  
negative.code:    "fields" : ["answer_count"] <1>
     "index_constraints" : { <2>
        "creation_date" : { <3>
           "max_value" : { <4>
              "gte" : "2014-01-01T00:00:00.000Z"
           },
           "min_value" : { <4>
              "lt" : "2015-01-01T00:00:00.000Z"
           }
        }
  