[gql] histogram interface

[mikeldking]

• Support numerics and categorical bins
• Support potentially a 3rd dimension
• Flexible enough for formatting on the UI
• Support for null bins

Numeric

[{ 
   start: 0
   start_inclusive: bool
   end: 1
   end_inclusive: bool
   value: 23
   unit: "%"
   null_bin: bool,
   additional_metric {
       value: 23,
       metric: "accuracy"
   }
}, {
  start: null
   start_inclusive: bool
   end: null
   end_inclusive: bool
   value: 23
   unit: "%"
   additional_metric {
       value: 23,
       metric: "accuracy"
   }
}]

Categorical

[{ 
   start: null
   start_inclusive: bool
   end: null
   end_inclusive: bool
   key: "wells fargo"
   value: 23
   unit: "%"
   additional_metric {
       value: 23,
       metric: "accuracy"
   }
}]