Why Istio's Metrics Merging Breaks in Multi-Container Pods (And How to Fix It)

If you run multi-container pods under Istio with STRICT mTLS, you're probably missing metrics And you might not know it. The containers are healthy. The scrape job shows no errors. But half your metrics are just... absent from Prometheus. No alert, no obvious explanation. I spent a while debugging this before I understood what was going on, so here's the full picture. The problem Istio has a built-in metrics-merging feature that lets Prometheus scrape a pod through the Istio proxy without reaching each container directly. It's useful. But it has a hard limitation that the docs mention only in passing: Istio's metrics-merge only supports one port per pod. The Superorbital team wrote the definitive explanation of why this is the case. The short version: Istio's proxy forwards the scrape to a single application port. If you have three containers each exposing /metrics on different ports, Istio picks one and ignores the rest. Someone opened a feature request for multi-port support back in