anchor.diff:  <<date,`date`>> or <<date_nanos,`date_nanos`>> field containing a timestamp. If
   you're rolling up a backing index or using the {ecs-ref}[Elastic Common Schema
   (ECS)], we recommend using `@timestamp`.
  ++
  +WARNING: Do not use this field in `metrics`. If you do, the rollup attempt will
  +fail.
   
   `calendar_interval` or `fixed_interval`::
   (Required, <<time-units,time units>>)
  
positive.code: <<search-aggregations-metrics-max-aggregation,`max`>>,
  <<search-aggregations-metrics-sum-aggregation,`sum`>>,
  <<search-aggregations-metrics-avg-aggregation,`avg`>>, and
  <<search-aggregations-metrics-valuecount-aggregation,`value_count`>>. You must
  specify at least one value.
  +
  NOTE: The rollup index stores these metrics in an
  <<aggregate-metric-double,`aggregate_metric_double`>> field. The `avg` metric
  
negative.code: `histogram`::
  (Optional, <<search-aggregations-bucket-histogram-aggregation,`histogram`
  aggregation>> object)
  Groups documents based on a numeric interval.
  +
  .Properties of `histogram`
  [%collapsible%open]
  ======
  `fields`::
  (Required*, string or array of strings)
  <<number,Numeric>> fields to group. `unsigned_long` fields are not supported. If
  you specify a `histogram` object, this property is required.
  +
  WARNING: Do not use these fields in `metrics`. If you do, the rollup attempt
  will fail.
  
  `interval`::
  (Required*, integer)
  