Allow terms agregation on _field_names field

[skundrik]

Would it be possible to allow terms aggregation (maybe others as well) on _field_names field in Kibana? I can do it in Sense by hand crafting the query. It would be useful for analysis of key=value type of messages.

GET index1/_search?size=0
{
  "query": {
    "query_string":{
      "query": "*"
    }
  },
  "aggs": {
    "Field names": {
      "terms": {
        "field": "_field_names",
        "size": 0
      }
    }
  }
}

[spalger]

You will be able to once #5806 is merged, by adding _field_names to the metaFields advanced config