[BUG] Searchable sidebar filter has no available fields

[ArgusLi]

Describe the bug

In the Discover page, when using an Index as a datasource, selecting 'Searchable' as 'yes' when Filtering by Type returns no available fields in the sidebar. This bug happens for PPL and SQL. When the same index is used in index pattern, there are available fields.

To Reproduce
Steps to reproduce the behavior:

1. Go to Discover.
2. Select an Index as a Datasource.
3. Filter by Type in the sidebar.
4. Set 'Searchable' to 'yes'.
5. Observe that there are no available fields in the sidebar.

Expected behavior
Available fields in the sidebar.

OpenSearch Version
v3.0.0

Dashboards Version
v3.0.0

Plugins

None

Screenshots

Host/Environment (please complete the following information):

• OS: macOS
• Browser and version: Electron v106