Selects series from a dataset (might contain multiple tag groupings). When name is provided, selects
that single channel. When name is omitted, selection starts from all channels in the dataset scope;
callers narrow the selection with a filterByTag predicate on the reserved NOMINAL_CHANNEL tag key
and preserve per-channel identity in the response with selectTags. Providing both name and a
NOMINAL_CHANNEL filter is rejected as an invalid request. Raw and derived channels can both be selected
through the reserved tag filter.