Instrumenting an LLM router: what New Relic sees, Prometheus doesn't

While routing prompts to different LLM tiers optimizes costs, it introduces complex risks—such as model drift, legal liability, and data privacy leaks—that standard "service up" metrics fail to detect. The author demonstrates how to mitigate these issues by building a three-tier local routing stack and leveraging New Relic to achieve the deep, automated observability necessary to monitor and fix routing decisions in real time.