At our organisation running a large fleet of .NET services, a deceptively simple question can be surprisingly hard to answer: what versions of .NET are our apps actually running in production? You'd think this would be easy. It isn't. Services get deployed, teams move on, and before long nobody is quite sure whether that one legacy service is still on .NET 6 — or even .NET Core 3.1. Spreadsheets fall out of date. README files lie. The only source of truth is what's actually running. We solved this with three lines of OpenTelemetry configuration. The problem We run dozens of .NET services across multiple teams. We are the middle of a push to .NET 10, but we have no reliable, centralised way to see the current state. We wanted to answer questions like: Which services are still on end-of-life .NET versions? Which teams still have work to do? After a migration wave, how do we confirm everything moved? The solution We already had OpenTelemetry set up ac...