Option to allow docvalue_fields of date types to return ms

[awick]

Prior to ES 6 docvalue_fields on dates returned ms, now it returns a formatted date. Would be nice if that formatting could be specified, including allowing epoch_millis.

https://www.elastic.co/guide/en/elasticsearch/reference/current/date.html
Says that "The first format will be used to convert the milliseconds-since-the-epoch value back into a string.", so maybe this is a bug and not a feature request, but it doesn't seem to work.

[awick]

Code to reproduce

curl -H "Content-type: application/json" -XPUT http://localhost:9200/bug27740 -d '
{
"settings" : { "number_of_shards" : 1 },
  "mappings" : {
    "type1" : {
      "properties" : {
        "field1" : {
          "type" : "date",
          "format" : "epoch_millis"
        }
      }
    }
  }
}'

curl -H "Content-type: application/json" http://localhost:9200/bug27740/type1/1 -d '{"field1": 10}'
curl http://localhost:9200/bug27740/_flush
curl 'http://localhost:9200/bug27740/_search?pretty&docvalue_fields=field1'

ES 5 results:

        "fields" : {
          "field1" : [
            10
          ]
        }

ES 6 results:

       "fields" : {
          "field1" : [
            "1970-01-01T00:00:00.010Z"
          ]
        }

[andyb-elastic]

@elastic/es-search-aggs