[RFE] Support Prometheus remote_write / remoteWrite in OCP 4.X

1. Proposed title of this feature request

[RFE] Support Prometheus remote_write / remoteWrite in OCP 4.X

2. What is the nature and description of the request?

Prometheus allows setting remote_write in its configuration [1] which allows it to send stats to external sources, such as other Prometheus, InfluxDB or Kafka.

The latest versions of the Prometheus Operator already implements this, albeit in an experimental way [2]

[1] https://prometheus.io/docs/prometheus/latest/configuration/configuration/#remote_write
[2] https://github.com/coreos/prometheus-operator/blob/v0.38.0/Documentation/api.md#remotewritespec - "If specified, the remote_write spec. This is an experimental feature, it may change in any upcoming release in a breaking way."

Version-Release number of selected component (if applicable):
4.x

Additional info:
For further details of how to configure this, see: https://access.redhat.com/solutions/4931911

3. Why does the customer need this? (List the business requirements here)

A customer currently uses a centralized InfluxDB instance to aggregate all the metrics of their Data Center and they would like to be able to send OCP stats, gathered by Prometheus, to their centralized Influx DB.

Hence, they would like the Prometheus remoteWrite config to be moved from "experimental" to fully supported by OCP 4, in this case, by the Openshift Monitoring Operator. This also means that the feature in the future can not change in a breaking way.

4. List any affected packages or components.

Monitoring oeprator, Prometheus operator