name: Country1; temperature: 22
name: Country1; temperature: 24

name: Country2; temperature: 24
name: Country2; temperature: 22

name: Country3; temperature: 25
name: Country3; temperature: 27

country1; average: 23
country2: average: 23
country3: average 26

AvgTemp23: 2 countries
AvgTemp26: 1 country.

GET /country/_search
{
   "size" : 0,
   "aggs": {
      "countryInfo": {
         "terms": {
            "field": "countryId"
         },
         "aggs": {
            "avg_temp": {
               "avg": {
                  "field": "temperature"
               }
            }
         }
      }
   }
}

"countryInfo": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": 1,
          "doc_count": 2,
          "avg_temp": {
            "value": 23
          }
        },
        {
          "key": 2,
          "doc_count": 2,
          "avg_temp": {
            "value": 23
          }
        },
        {
          "key": 3,
          "doc_count": 2,
          "avg_temp": {
            "value": 26
          }
        }
      ]
    }