This is the doc story for this issue: https://issues.redhat.com/browse/OSSM-566.

As discussed, this could be documented in two steps:

1. Configuring Istio (within OSSM) to use OpenShift (or External) Prometheus, but excluding Istio. This is what jewertow@redhat.com has previously demoed using the Prometheus extension provider (GA in 2.4).
2. Configuring all of OSSM (including Kiali) to use OpenShift (or External) Prometheus). This issue may be related or it may be too old: https://issues.redhat.com/browse/OSSM-3184.

Navigation: OpenShift Container Platform > Service Mesh > Service Mesh 2.x > Metrics, logs, and traces > Integrating with user-workload monitoring

Direct link to content: https://docs.openshift.com/container-platform/4.12/service_mesh/v2x/ossm-observability.html#additional-resources_user-workload-monitoring 

Also being updated via https://issues.redhat.com/browse/OSSM-4565 to include configuring Kiali for user-workload monitoring.