anchor.diff:  `field`::
   (Required, string)
   <<number,Numeric>> field to collect metrics for.
  ++
  +WARNING: Do not use the same fields in `histogram` and `metrics`. If you specify
  +the same field in both `histogram` and `metrics`, the rollup attempt will fail.
   
   `metrics`::
  -(Required, array of strings)
  -Array of metrics to collect. Each value corresponds to a
  +(Required, string or array of strings)
  +Metrics to collect. Each value corresponds to a
   <<search-aggregations-metrics,metric aggregation>>. Valid values are
   <<search-aggregations-metrics-min-aggregation,`min`>>,
   <<search-aggregations-metrics-max-aggregation,`max`>>,
  
positive.code: [%collapsible%open]
  ======
  `fields`::
  (Required*, array of strings)
  Array of <<number,numeric>> fields to group. If you specify a `histogram`
  object, this property is required.
  
  `interval`::
  (Required*, integer)
  
negative.code: [%collapsible%open]
  ======
  `fields`::
  (Required*, string or array of strings)
  <<keyword,Keyword family>> and <<number,numeric>> fields to store. If you
  specify a `terms` object, this property is required.
  +
  TIP: Avoid storing high-cardinality fields. High-cardinality fields can greatly
  increase the size of the resulting rollup index.
  