Use _doc_count data type when indexing histograms

[dgieselaar]

In the APM app, we are currently using value_count aggregations for transactions, as transaction metric documents use histogram fields, meaning doc_count is no longer representative. ES now also has a _doc_count data type that allows us to specify how many documents a transaction metric document represents. ES will then use this field in its bucketing aggregations, removing the need for an additional value_count aggregation, which should speed up some of our queries and remove some complexity from our queries.

https://www.elastic.co/guide/en/elasticsearch/reference/master/mapping-doc-count-field.html

[axw]

I think this should be straightforward. I thought we would need to update betas and fields.yml; that is not the case.

To whomever implements this: I think we'll just need to add a field describing the doc count to model.Metricset, and use that to add a _doc_count field to the metric event docs that we index.

[jalvz]

Checked _doc_count field in metrics matched the number of docs running the aggregation:

GET apm-7.12.0-transaction/_search
{
  "aggs": {
    "txnames": {
      "terms": { "field": "transaction.name" } 
    }
  }
}