Elasticsearch term aggregation to regroup a field (key) and count the number of buckets using cardinality aggregation

elasticsearch_term_aggregation_bucket_count_cardinality.md

GET rule_index/_search
{
  "size": 0,
  "aggs": {
    "rule_doc_count": {
      "terms" : { "field" : "key.keyword" }
    },
    "rule_count":{
      "cardinality": {
        "field": "key.keyword"
      }
    }
  }
}