Add option to adjust _source options + improvements

[tsg]

This PR adds/fixes:

• Ability to disable _source, or set other _source related options
• Fixes the overwrite logic (was using the wrong template name on the check)
• Fixes error handling
• Integration tests for overwritting

The source can be disabled like this:

setup.template:
  settings._source.enabled: false

And _source can be also partially enabled via "includes" and "excludes", for example:

setup.template:
  settings._source.excludes: ["metricset", "system.process.cmdline"]

Adding the discuss label for naming. I hesitated between using _source or source_settings for these options, but I like the look of _source.enabled: false.

Part of #3654 and #4112.

Remaining TODOs:

• Update docs
• Changelog

[ruflin]

LGTM

[ruflin]

At first I was also sceptical about using _source but I think for ES users it must be quite intuitive to use it like this.

[ruflin]

Seeing both names I was thinking if it could be settings._source and settings.x but I would not know how to call x :-(

Is there are stuff we potentially going to add here?

[tsg]

I can imagine there could be more yes (e.g. _all), although I don't plan more at the moment. We could maybe use index for x, so you'd have: settings.index.number_of_shards and settings._source.enabled: false. Trying to figure out if there can be any settings that do not fall under index...

[tsg]

I've updated the code to have settings.index for the global index settings and settings._source for the _source settings. Elasticsearch is a bit lenient when it comes to the index settings in the template. Both of these work:

"settings": {
  "number_of_shards": 1
}

and

"settings"; {
  "index": {
    "number_of_shards": 1
  }
}

With this change, we are can only use the second version. I think that's good anyway, as we don't want two ways of accomplishing the same thing.

[elasticmachine]

[golint] _{reported by reviewdog 🐶}
exported type TemplateSettings should have comment or be unexported

[elasticmachine]

[golint] _{reported by reviewdog 🐶}
type name will be used as template.TemplateSettings by other packages, and that stutters; consider calling this Settings

[ruflin]

I like the new structure with the index prefix of the settings. Left a minor docs comment.

[ruflin]

Should this be updated to put index on the line above?

[tsg]

I kind of like it like it is now because it matches the Elasticsearch docs where they usually talk about index.* as the name of the options: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html