Hi, awesome project and a pleasure to use! Would love to be able to use variables in alert metrics :-)
It's also frustrating that it's effectively impossible with the Cloudwatch data source to select resources to alert on using their tags (e.g. select the elb with app=foo and env=prod). The dimension_values() function lets you filter based on tags, but instead of being usable directly in a panel's query it requires using a variable as an intermediary, which then disables alerting.