Usage returned after recording one streamed model message.
Input-token delta contributed by this message.
Output-token delta contributed by this message.
Cost contributed by this message, or None when pricing was unavailable.
Negative when the message re-prices its request downward -- a corrected prompt count, or a model that turned out to cost less than the fallback the earlier chunks were priced against.
Running token total for the request after applying this message.
