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

Istio's built-in metrics-merging feature only supports scraping a single application port per pod, causing metrics from additional containers to silently disappear from Prometheus without any error. Common workarounds like direct port scraping or multiple PodMonitors fail under STRICT mTLS because all traffic must pass through the sidecar proxy. Ambient mode (GA since Istio 1.24) eliminates this issue by removing the per-pod sidecar, but most production deployments still run sidecar mode.