Got it. Tags can be of the name:value kind. for ex: role:db or availability-zone:us-east-a1 or foo:bar

And you can then ask for "my.metric" summed over "availability-zone:us-east-1a" grouped by "role".

Here's a screenshot to show what it looks like https://skitch.com/oliveur/emywc/sobotka-datadog

You obviously have a lot of experience with monitoring tools - I'd love to connect. I'm @oliveur on the twitters.