List Current Pricing

List the rate each priced model is metered at, one row per model key.

Listing prices answers the stored history, one row per effective_at, so a page of that is a page of revisions rather than a page of models. This answers one row per key: the newest rate that has taken effect, or the earliest scheduled rate for a key that has none yet. count is the number of priced models, so a caller can page without reading the collection to learn how long it is.