when testing a host/metric, it'd be nice to be able to specify a metric regex instead of metric name. Some metrics prepend info like processname (eg nginx_error_count vs. apache_error_count) and it'd be nice to say look for .*_error_count instead of writing two checks.

In the callback mode, it'd be nice to specify a host substring or regex to match instead of triggering the callback on all hosts. Maybe you only need disk monitoring on your db hosts but you want to use the callback method. Right now, the responsibility lies in the person writing the callback to do host selection; it would be easier to write checks if it were clear how to pass that restriction in to the library.