The metrics developer docs say "metrics are tagged by key value pairs, where each key can have more than one value." Also the RFC says "More than one value per key is permitted if represented as a list or tuple." Multi-valued tags is also implemented in the python sdk (not sure about the other sdks).

However, it seems that relay doesn't allow multi-valued tags for single emissions, only the last value of the list/tuple sent is actually used. Have raised this issue offline but opening it here so it's not forgotten.