Looks like you need this:
${restadmin("boards/id/${coreNode.id}/metrics/name/avg_thread_response_time?date_start=2023-01-01&date_end=2023-02-01").value}
I might picked the wrong metric, but you can find the name of each by grabbing the value from url like
/t5/bizapps/page/tab/community%3Aadmin%3Aanalytics-main%3Aadvanced-metrics?board_selection=freelancers&date_range=3-12-2023%2C4-12-2023&granularity=Daily&metrics=avg_thread_response_time&metrics_scope=board&user_selection=Empty
Dates can get weird, but you should be able to use the datesupport method
datesupport.addMonth(-1)
Does this calculate "total" or "average" for you? I ran into this issue again and this API metric is definitely still giving me the total 🤦
