Composite metrics

The use case with tagged metrics is fairly simple, though recipes get much more complicated when you need to search for check bundles from other servers (determining check_id's is non-trivial at the moment).

I thought it was better to leave this documentation simpler, and if someone needs to find check_ids to manually combine checks from other servers, leave that as an exercise in refactoring the MetricScanner. Maybe a BundleScanner?