[Lens] Fields shown as empty in some cases

[flash1293]

Kibana version: master

Describe the bug: In some cases fields are shown as empty even though they contain data

Steps to reproduce:

1. Ingest these documents:

POST testindex/_doc
{
  "ts1": "2020-10-25",
  "ts2": "2020-10-25",
  "val": 5,
  "val2": [5,4]
}

POST testindex/_doc
{
  "ts1": "2020-10-24",
  "ts2": "2020-10-24",
  "val": 2,
  "val2": 9
}

2. Create an index pattern with time field on "ts1"
3. Go to Lens and adjust time range to include the time range
4. All fields shown as empty

Expected behavior:
Fields should be shown as existing

Screenshots (if relevant):

Any additional context:
I previously thought this would be caused by scripted fields (see #84194 ), but I can't reproduce this behavior anymore. Maybe there's just a race condition of some kind going on in general.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[mattkime]

This is likely the same bug as #84194 - introduced by #82223

relevant code - x-pack/plugins/lens/server/routes/existing_fields.ts:139

To resolve - use index pattern api to load field list instead of reading raw field list. This is also important for index pattern defined runtime field support.

[nreese]

Until #84659 is merged, to view the problem when using sample data, under index pattern management - add a custom label to a field. This will cause the index-pattern saved object to be updated and remove fields cache.