[3.13] Add documentation for token usage in streaming responses

[tomek-labuk]

Context: https://docs.google.com/document/d/1zoKNEcVvUNsmupPmOeG3HUzFCoWCOQR7_juW_SWYOpc/edit?tab=t.0#heading=h.80hogtdv6agg

DoD; Update AI Gateway streaming document