njaln: arm and metrics have different purposes. arm tells you more about your relay/client. the metrics portal tells you more about the network including historical data.
i'm not sure what results there are in arm that might be useful for the metrics portal. but some of arm's data depends on the local view on the network, which may not be available for all relays.
see the metrics website for some examples of results. there are some graphs, there is a data page with possibly more results, and then there's dir-spec.txt which specifies more data we might use for analysis.
and then there's data that we gather ourselves, e.g. by active measurements.