[API] Add metafields from uiSettings to index patterns created by ingest API

[Bargs]

Index patterns created via the ingest API weren't getting meta fields (_source, _id, etc.) saved in the index pattern's fields cache. This led to "unknown" fields showing up in Discover.

Now that #6849 is merged, I've re-worked this code to use the uiSettings module to grab the user configured (or default) metaFields and add them to any index pattern created via the ingest API.

How to test

POST to /api/kibana/ingest with a request payload like the following:

{
    "index_pattern": {
          "id": "logstash-*",
          "title": "logstash-*",
          "time_field_name": "@timestamp",
          "fields": [{
            "name": "ip",
            "type": "ip"
          }, {
            "name": "@timestamp",
            "type": "date"
          }, {
            "name": "agent",
            "type": "string"
          }, {
            "name": "bytes",
            "type": "number"
          }]
    }
}

In addition to the fields listed in the payload, the index pattern should also include any meta fields currently configured in the ui settings.

[Bargs]

jenkins, test it

[bevacqua]

Consumers of the uiSettings shouldn't need to know internals like this. I suggest we add a .get(key) method like:

key => getAll().then(config => _.get(config, `${key}.userValue`, config[key].value));

Code here becomes:

const metaFields = await server.uiSettings().get('metaFields');

[Bargs]

I agree, I had to do a lot of trial and error to figure this out.

[bevacqua]

@Bargs PR looks great other than minor comments 🎉

[Bargs]

@bevacqua I updated the code to use an implicit return and pushed the change. Do you mind if we merge this prior to adding the simplified getter to uiSettings and then updated it once it's in? I'd like to get this one last PR merged into feature/ingest before putting up the feature/ingest PR for merging to master.

[bevacqua]

LGTM