[MODEL] Added functionality to provide custom value of total count for pagination

[bk-az]

The Problem

When using collapse parameter to collapse search results, the total number of hits in the response indicates the number of matching documents without collapsing. As a result, the total number of pages for pagination is miscalculated.

Assume, for example, the following query on the group_items index returns a total of 100 results without collapsing.

GET group_items/_search
{
  "query": {
    "match_all": {}
  }
}

And when the search results are collapsed based on the group_id field value. The total results returned by the query becomes only 5 instead of 100.

GET group_items/_search
{
  "query": {
    "match_all": {}
  },
  "collapse": {
    "field": "group_id"         
  }
}

The problem occurs when we paginate results, after collapsing, the value of the total number of hits is still 100 in response, which results in the miscalculation of the total number of pages, as mentioned in the reference:

The total number of hits in the response indicates the number of matching documents without collapsing. The total number of distinct groups is unknown.

The solution

This problem with pagination can be solved by providing a custom value of total_entries (will_paginate) or total_count (kaminari). The value of total distinct groups can be calculated by using cardinality aggregation

# Assuming a method `total_collapsed` is defined to get the total count of 
# collapsed results, using cardinality aggregation.
total_count = total_collapsed(response.aggregations)

Now to get pagination working correctly, the value of total_count can be specified.

# for will_paginate
response.paginate(page: 1, per_page: 30, total_entries: total_count)

# for kaminari
response.page(1).total(total_count)

[bk-az]

@picandocodigo can u please review this PR?