TSVB doesn't communicate it's index-patterns to dashboard

[Dosant]

Kibana version: 7.9.3 / master. Works in 7.6.1. Regressed somewhere between 7.6.1 -> 7.9.3

Describe the bug:

For dashboard's filter bar to work visualisations communicate its index-patterns.
Some visualisations don't do that (#19408)
But according to (#19408 (comment)) TSVB should.

This is causing bugs and confusions with how filter bar is working on a dashboard when TSVB is used.
Especially filter bar is almost non usable for dashboards that have only TSVB visualisations and use non-default index patterns.

Steps to reproduce:

1. Create a TSVB with non default index pattern
2. Create a dashboard with single TSVB lib
3. Try to use a filter bar and notice that there is no way to use pick a filter for used index pattern

Expected behavior:

I should be able to create a filter for TSVB's index-pattern.

Any additional context:

• I checked that TSVB properly communicated it's index-patterns in 7.6.1.
• Code in dashboard which gathers index-patterns from panels: code
• Related: Improve experience on dashboard when visualizations do not use index patterns created in Kibana #19408

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)