_size does not support doc_values

[pickypg]

Currently, if you try to create a mapping that tries to set "doc_values" : true, then it will either ignore the _size field or ignore that setting:

PUT /test
{
  "mappings": {
    "type": {
      "_size": {
        "enabled": true,
        "doc_values" : true,
        "store": true
      }, 
      "properties": {
        "field": {
          "type": "string"
        }
      }
    }
  }
}

This results in:

GET /test/_mapping
{
  "test": {
    "mappings": {
      "type": {
        "_size": {
          "enabled": true,
          "store": true
        },
        "properties": {
          "field": {
            "type": "string"
          }
        }
      }
    }
  }
}

After adding some documents, then sorting on _size, you can see that it does indeed ignore the doc values setting (rather than just not showing it set, which was unlikely):

PUT /test/type/1
{
  "field" : "value1"
}

PUT /test/type/2
{
  "field" : "value2"
}

GET /test/_search
{
  "fields": ["_size"],
  "sort": [
    {
      "_size": {
        "order": "desc"
      }
    }
  ]
}

Fielddata:

GET /_cat/fielddata?fields=*&v
id                     host        ip           node   total  _size 
2xF59inESXKOIkzK7GkavA my-hostname 192.168.1.31 Kl'rt 16.5kb 16.5kb 

This is not critical as _size must be enabled to even be used, but in that case, it would be useful to make use of doc values.

[rjernst]

In #8143 we locked down meta fields, so even store here is no longer possible to set (it is always stored). It is important the configuration of meta fields are managed, so that we can depend on them for features. Now that doc values are enabled by default for 2.0, for non-tokenized and indexed fields, _size will have doc values when enabled.