Enable aggregatable for special fields in index pattern

[monicasarbu]

Use keyword for special fields(_index, _id, _type and _score). This depends on #3282.

[ruflin]

LGTM.

@blacktop Can you have also a quick look?

[blacktop]

So the way I fixed it was to add this: https://github.com/blacktop/brobeat/blob/master/scripts/generate_index_pattern.py#L99

Which is over-kill as your solution is much simpler, however, the only thing that is different is that the _type field has the following set.

    meta_fields = [
        {
            "name": "_id",
            "type": "string",
            "count": 0,
            "scripted": False,
            "indexed": False,
            "analyzed": False,
            "doc_values": False,
            "searchable": False,
            "aggregatable": False,
        },
        {
            "name": "_type",
            "type": "string",
            "count": 0,
            "scripted": False,
            "indexed": False,
            "analyzed": False,
            "doc_values": False,
            "searchable": True, <===========================
            "aggregatable": True, <===========================
        },
        {
            "name": "_index",
            "type": "string",
            "count": 0,
            "scripted": False,
            "indexed": False,
            "analyzed": False,
            "doc_values": False,
            "searchable": False,
            "aggregatable": False,
        },
        {
            "name": "_score",
            "type": "number", <=======================
            "count": 0,
            "scripted": False,
            "indexed": False,
            "analyzed": False,
            "doc_values": False,
            "searchable": False,
            "aggregatable": False,
        },
    ]

NOTE: I set all the default fields in my example (which I know the python will set automatically in your solution)

Also, I think the defaults will make the meta-fields, indexed, doc_values, searchable and aggregatable all True, but Kibana makes them all False by default as can be seen above.

This can been seen in your code here - https://github.com/monicasarbu/beats/blob/53e873325a51bfaa85b78b48b2e3bf3f32538bb6/libbeat/scripts/generate_index_pattern.py#L44

    field = {
        "name": path,
        "count": 0,
        "scripted": False,
        "indexed": True, <=======================
        "analyzed": False,
        "doc_values": True, <=======================
        "searchable": True, <=======================
        "aggregatable": True, <=======================
    }

Also kibana makes _score a number and not a integer, but I am not sure that matters.

[monicasarbu]

@blacktop I have updated the PR to set the fields (_index, _id, _score and _type) with the same options as Kibana. Thanks for the detailed feedback. I am setting _score of type integer as the script changes an integer to number.