[ML] Data Frames Does Not Show All Index Fields

[aleph-zero]

Kibana version: 7.3
Elasticsearch version: 7.3
Server OS version: N/A (Elastic Cloud)
Browser version: Chrome
Browser OS version: MacOS
Original install method (e.g. download page, yum, from source, etc.): Elastic Cloud
Describe the bug:
When creating a new data frame, the "Source index" table does not show all fields in the "Select Fields" drop down.

You can see the list of fields in the image. However, if I look at the index in the Discover app, then I see many more fields.

I want to choose some of the fields for my transform, such as destination.port, but cannot. I got some of these extra fields from the iptables filebeat module. Maybe the data frames UI cannot read these fields?

Steps to reproduce:

1. Install filebeat
2. Enable iptables module
3. Try to create a data transform

Expected behavior:
Should show all fields from all enabled filebeat modules.

[elasticmachine]

Pinging @elastic/ml-ui

[LucaWintergerst]

The same issue is also present for the Data Frame Analytics => Analytics Exploration UI, when viewing the result of an outlier detection job

[walterra]

Related to #49946.

[walterra]

Fixed in #63447. The columns available in the source preview table are now based on Kibana index pattern fields.