anchor.diff:        max(a_f),
         avg(a_i),
         avg(a_f),
  +      std(a_f),
         per(a_f, 50),
         per(a_f, 75),
         count(*))
  
positive.code: This value is applied to each dimension.
  '-1' will fetch all the buckets.
  * `metrics`: List of metrics to compute for the buckets.
  Currently supported metrics are `sum(col)`, `avg(col)`, `min(col)`, `max(col)`, `count(*)`, `per(col, 50)`.
  The `per` metric calculates a percentile
  for a numeric column and can be specified multiple times in the same facet function.
  * `tiered`: (Default true) Flag governing whether the `facet` stream should parallelize JSON Facet requests to multiple Solr collections using a `plist` expression; this option only applies if the `collection` is an alias backed by multiple collections.
  
negative.code:       max(a_f),
        avg(a_i),
        avg(a_f),
        std(a_f),
        per(a_f, 50),
        per(a_f, 75),
        count(*))
  