This was added: https://github.com/GoogleCloudPlatform/prometheus-engine/blob/v0.12.0/doc/api.md#exportspec
You can access the operatorconfig:
kubectl get -n gmp-public operatorconfig/config
NAME AGE
config 165d
Here is the example they provided: https://github.com/GoogleCloudPlatform/prometheus-engine/blob/v0.12.0/examples/export-write/README.md