[TSVB] When comma seperated index patterns contain a missing index, the fields dropdowns are not populated.

[simianhacker]

Kibana version:

7.9BC and Master

Elasticsearch version:

7.9BC and Master

Describe the bug:

When the Index pattern field under Panel Options has multiple patterns and one of them doesn't exist, ALL the drop down fields are empty. This issue became a problem when the Metrics UI upgraded their default index pattern from metricbeat-* to metrics-*,metricbeat-* in preparation for indices created by the new ingest manager. .

• This may also be related to [TSVB] Multiple Index Patterns #67754
• This was originally discovered in [Metrics UI] Open in visualize no longer works in Metrics Explorer #72529
• We've created a temporary work around on the Metrics UI with [Metrics UI] Fix Metrics Explorer TSVB link to use workaround pattern #73986

Steps to reproduce:

1. Index some data from Metricbeat
2. Create a new TSVB visualization
3. Change the Index pattern under Panel options to metrics-*,metricbeat-*
4. Attempt to select @timestamp under the Time field

Expected behavior:

It should populate the dropdown fields with the fields from GET _field_caps?index=metrics-*,metricbeat-*&fields=*

[elasticmachine]

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