Variable settings from loki data source

I am currently setting variables in the dashboard, and I use the loki data source, but the query type on the page can only select the label, because I want to extract the value of a certain field under my msg as my variable, which makes it difficult for me to achieve it. Is there any good solution at present?
For example: My logs Kanban statement {service_name=“hello”}| json | groupId="… " , This is the form of logs extraction. How to achieve it in variable setting