Fix anthropic saturation points to take into account multiple services

GitLab Infrastructure Service - incident.io
2026-03-31T10:03:12Z

Both AI-gateway and Duo Workflow Service use the same antropic quota and emit metrics. But in the saturation point, we aren't aggregating them: https://gitlab.com/gitlab-com/runbooks/-/blob/fce35dfee015cae58484b59d4e50ad104d60b753/libsonnet/saturation-monitoring/token_usage.libsonnet#L11

_This ticket was created from_ [_INC-8876_](https://app.incident.io/gitlab/incidents/8876) _using_ [_incident.io_](https://app.incident.io) 🔥