How to get latest values for each group with an Elasticsearch query?

For those like user1892775 who run into "Fielddata is disabled on text fields by default...", you can create a multi field (https://www.elastic.co/guide/en/elasticsearch/reference/current/multi-fields.html). So you might have mapping like:

"mapping": {
    "properties": {
      "country": {"type": "string", "fields": {"raw": {"type": "string", "index": "not_analyzed"}}}
}

Then your query would look like

POST /test/_search?search_type=count
{
    "aggs": {
    "group": {
        "terms": {
            "field": "country.raw"
        },
        "aggs": {
            "group_docs": {
                "top_hits": {
                    "size": 1,
                    "sort": [
                        {
                            "collected": {
                                "order": "desc"
                            }
                        }
                    ]
                }
            }
        }
    }
  }
}

(Note the use of country.raw)

You can use a top_hits aggregation that groups on the country field, returns 1 doc per group, and orders the docs by the collected date descending:

POST /test/_search?search_type=count
{
    "aggs": {
        "group": {
            "terms": {
                "field": "country"
            },
            "aggs": {
                "group_docs": {
                    "top_hits": {
                        "size": 1,
                        "sort": [
                            {
                                "collected": {
                                    "order": "desc"
                                }
                            }
                        ]
                    }
                }
            }
        }
    }
}