GMP: Support for Prometheus Remote Write

Hi Folks,

My team is considering moving over to [prometheus-engine](https://github.com/GoogleCloudPlatform/prometheus-engine) and is currently using [prometheus-operator](https://github.com/prometheus-operator/prometheus-operator) as that allows us to use [the remote-write-receiver flag](https://prometheus.io/docs/prometheus/latest/feature_flags/#remote-write-receiver). Can anyone provide a view as to when a feature like the remote-web-receive flag used by vanilla prometheus might be exposed by [prometheus-engine](https://github.com/GoogleCloudPlatform/prometheus-engine)?

Kind regards,
Edward