TSVB field list performance issue on using annotations

[flash1293]

Kibana version: 7.10

Describe the bug: When adding an annotation datasource to TSVB, the index input is filled with * by default.
This causes two requests to /api/metrics/fields?index=* which will fetch fields of all indices accessible by the current user and send them to the client. Depending on the amount and variety of indices this can be a relatively costly operation, choking the responsiveness of the Kibana server and client.

This is further amplified by additional requests on each keystroke while typing in the input field

Steps to reproduce:

1. Go to TSVB
2. Switch to Annotation tab and add datasource
3. Request is sent twice for * index pattern
4. Start typing
5. Request is sent for each keystroke

Expected behavior:

I can think of two different ways to improve this:

• Set the input to empty string by default and don't send a request for fields at all (require the user to enter an indexpattern first) - the "*" default is not a good default anyway because it's very rarely the right choice
• Debounce responsiveness of the input - don't fetch fields on each keystrokes, only after the user stopped typing for 0.5s or something similar

[elasticmachine]

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

[alexwizp]

@flash1293 I agree with you, I think we should implement both of your suggestions. We already use debounce for the data request, and I have no idea why it's not working for fields.... it should be checked. Replacing * to the default index looks like a good performance improvement for me.