[indexPattern] added _score as a default meta field

[spalger]

While looking into #4012 I noticed that you can't even select _score as a sort field right now. This is because the doc_viewer checks that the sorts field belongs to the indexPattern. This is a generally good behavior, so I figured it would make sense to add _score as one of the default _metaFields.

For now, you can see this in action by opening up a fresh instance of discover and changing the sort field to _score

[w33ble]

When using an index pattern that was created before the changing, handling of the _score field is a little odd, since the field doesn't exist in the mapping, but does show up in Discover

How much of a problem this is though, I don't know.

[w33ble]

Not a show-stopper, but adding an exception by field name here is unfortunate. Since this field is being forced into the mapping, it seems like there should be a property we could set that would indicate that it's sortable.

[spalger]

I agree with you in principle, but there really is no other field like _score (in that they are always sortable) and I feel like whatever property we add to the spec will equal spec.name === '_score' anyway.

[w33ble]

Overall this LGTM, just concerned about the handling of _score for existing index patterns so I wanted to pass it back to get a response on that.

[spalger]

Opened #4256 to track you ask about adding _score to existing patterns.

[spalger]

Merged master, once the tests pass I'll merge this.