We will keep this abstractions as it's convenient, otherwise IngestDocument would depend on ScriptService directly, and would explicitly rely on mustache which is not even part of core. better to have the interface in core, and the impl as part of the ingest plugin, which relies on mustache, shipped with core by default.
The append processor allows to append one or more values to an existing list; add a new list with the provided values if the field doesn't exist yet, or convert an existing scalar into a list and add the provided values to the newly created list.
This required adapting of IngestDocument#appendFieldValue behaviour, also added support for templating to it.
Closes#14324
Added ingest wide template infrastructure to IngestDocument
Added a TemplateService interface that the ingest framework uses
Added a TemplateService implementation that the ingest plugin provides that delegates to the ES' script service
Cut SetProcessor over to use the template infrastructure for the `field` and `value` settings.
Removed the MetaDataProcessor
Removed dependency on mustache library
Added qa ingest mustache rest test so that the ingest and mustache integration can be tested.
Martijn van Groningen
javanna
Tal Levy