We have received several requests to add "rate" functionality or aggregation: how many docs/s were there per bucket in a date_histogram? We have decided to implement this as a special metric aggregation with a scope limited to only date_histogram aggregation at the moment, in other words, the rate aggregation will have to be a descendant of a date_histgram and in the case of nested histograms the closest ancestor will be used to determine the rate.

POST /sales/_search
{
  "aggs": {
    "sales": {
      "date_histogram": {
        "field": "date",
        "calendar_interval": "day"
      },
      "aggs": {
        "the_rate": {
          "rate": {
            "unit": "second" 
          } 
        }
      }
    }
  }
}

{
  ...
  "aggregations": {
    "sales": {
      "buckets": [
        {
          "key_as_string": "2020-07-29",
          "doc_count": 300000,
          "the_rate": {
            "rate": 3.47222222222,
            "rate_as_string": "3.47222222222/s"
          }
        },
       ...
      ]
    }
  }
}

By default the number of documents in the bucket will be used to calculate the rate, but it will be also possible to specify a numeric field to use sum of all values of the field to calculate the rate:

      "aggs": {
        "the_rate": {
          "rate": {
            "field": "num_of_requests" 
            "unit": "second" 
          } 
        }
      }

We could also add support for "accumulative" : true flag to address #60619 in a future iteration.