[BUG]: Field dropdown in Color Theme section only takes numeric fields

[pratibhapandey16]

What is the bug?
Field dropdown in Color Theme section only takes numeric fields.

How can one reproduce the bug?
Steps to reproduce the behavior:
1.Launch OpenSearch Dashboard UI
Go to Observability->Event Analytics->Explorer
2. Enter PPL Query and select "Year to date" option from calendar
3. Click Refresh
4. Click on Visualizations tab
5. Select the Vertical bar chart option from chart selector dropdown
6. Click on '+ Add color theme' link
7. Select field from the field dropdown

What is the expected behavior?
The Field dropdown should have all the other fields along with numeric ones

Do you have any screenshots?

@anirudha @mengweieric
cc: @abasatwar @spattnaik