Unified reporting for all AI Gateway usage

Introduced the Custom Reporting API for AI Gateway in beta, enabling programmatic access to cost, token usage, and request volume across AI Gateway traffic, including BYOK requests. This allows for detailed breakdown of spend by model, provider, user ID, custom tag, or credential type, facilitating per-feature, per-customer, and per-pricing tier cost tracking from a single endpoint. The API supports tagging requests with 'user' and 'tags' for attribution, and can be queried via a GET request to `/v1/report`. The API captures data for both BYOK and system credentials, enabling operational metrics for AI spend.