Per-project LLM cost attribution with OTel spans: the wiring

TL;DR. If your LLM bill is one line item on a cloud invoice, you cannot answer "which team spent that." We fixed this by tagging every gateway span with team.id, project.id, and feature.id, plus the OpenInference token-count attributes, shipping those spans through an OTel collector into Tempo, and rolling cost up per team with TraceQL in Grafana. The payoff that sold it internally: one team's monthly spend quietly went from a few hundred dollars to over a thousand because of a retry loop, and t...