Return a dictionary for updating `last_metrics` with `mets`.

def add_metrics(last_metrics:Collection[Rank0Tensor], mets:Union[Rank0Tensor, Collection[Rank0Tensor]]):
    "Return a dictionary for updating `last_metrics` with `mets`."
    last_metrics,mets = listify(last_metrics),listify(mets)
    return {'last_metrics': last_metrics + mets}