I'm not able to find anything in the documentation on how to perform the below. How would I configure a health_check to look at disk/memory usage on an upstream server? Or that a database service is running?

http://nginx.com/products/application-health-checks/
Regular execution of health tests that run on the upstream servers that verify key services are connected and accessible (e.g. database, filesystem) and resources are not exhausted (disk, memory)