[Search] Improve meta fields handling

[kertal]

In Kibana Advanced setting Meta fields can be defined

Information about ES Meta fields: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-fields.html

This setting originated of times when fetching _source was the default. it's used to merge the meta fields of the result into the displayed document, so it's available for field selection, and visible in the Document column

In the given example the meta field _tier was added, but since it's not requested, no results are being displayed. This should be improved:

• meta fields like _tier that were added in Advanced settings should be requested, this is possible when using the fields param of the search https://www.elastic.co/guide/en/elasticsearch/reference/current/search-fields.html#search-fields-param
• we could consider adding _tier as default, since it can be valuable to get more insights in which tier a document is stored

Out of scope, but worth to think about

• Merging / using all underscore prefixed values of a returned document, those are few (a total of 10, 4 are returned as default), and why hiding potential useful information
  • Advantages, plugin based meta fields work out of the box, no configuration necessary
• Requesting additional meta values by default
• Removing the configuration, unless there's a case why any given meta value should not be available for selection, those are few,

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)