Users that have custom metrics in 2.0 or 1.1 cannot achieve the same result in 2.1 because there's no mixer anymore and we don't support a way to edit the EnvoyFilter. It's a regression.
Upstream documentation instructs users to edit the Istio Operator Custom Resource.
We could do the same with our SMCP. Charts are already prepared. We just need to expose .values.telemetry.v2.prometheus.configOverride in SMCP, under .spec.telemetry.