[FEATURE] added _doc_count field to summary documents

[petardz]

Is your feature request related to a problem?

We need this new field documented for v2.2.0 PR

Do you have any additional context?

Description would be something like this:

Documents that store pre-aggregated data will have _doc_countfield representing the number of documents aggregated in a single summary field. When computingdoc_countfield, value of_doc_countwill be used, if available, instead of1.
This could be added to bucket-agg.md in "doc_count" paragraph.

[cwillum]

@petardz To clarify, from your "additional context" description: "documents that store pre-aggregated data" are those calculated and shown in a response. Yes? Could I see an example of this?
And, this field appears as _doc_count and should not to be confused with doc_count Right?

[cwillum]

@petardz Actually, it might be more helpful to have a realistic example of a request that would naturally include this field for documents that store pre-aggregated data. Can you think of a document that that would include pre-aggregated data?

[petardz]

@cwillum
[removed external reference]

In ISM plugin, rollup jobs store these documents with pre-aggregated data in rollup indices. We need small PR to include this _doc_count field in them and once that's done we will have correct doc_count calculation when querying rollup indices.