[reporting] formatters are loaded from index pattern saved object, should be loaded via index patterns service

[mattkime]

#82223 removed fields from the index pattern saved object (except for scripted fields).

x-pack/plugins/reporting/server/export_types/csv/generate_csv/index.ts:86 loads the index pattern saved object, looks at the field list and loads all the field formatters.

pseudo code recommendation -

const fields (assume defined);
const indexPattern = await indexPatterns.get(id);
fields.reduce((collector, field) => {
  const indexPatternField = indexPattern.fields.getByName(field);
  collector[field] = indexPattern.getFormatterForField(indexPatternField);
  return collector;
}, {});

Its possible that the majority of the work will involve bootstraping the index patterns service.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[mattkime]

x-pack/plugins/reporting/server/export_types/csv/generate_csv/field_format_map.ts:47 might be a better starting point. The field list will no longer be available via the saved object except for scripted fields.

[tsullivan]

This issue may be closed out when CSV export switches to using SearchSource on the server rather than push index pattern info through the job params. That work is prioritized as it will be needed to support Runtime Fields with CSV Export: #81079

[tsullivan]

@mattkime I'm not 100% sure there was an issue here, but FYI this PR: https://github.com/elastic/kibana/pull/88303/files is re-writing CSV Export. The field list (which columns to show) comes from a serialized SearchSource object.

Mind taking a look at #88303 and see if that goes in the right direction?