How to show only taxonomy parent terms and hide child terms

Problem/Motivation

The taxonomy term is hierarchical, such as locations taxonomy:
Country
--State
The content type has a reference to this taxonomy with unlimited values. Hope the facets only show the parent terms: country, and hide the child terms. Is there a way to implement this? Thanks.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes