Add documentation on integration with Prom Operator

[allenlsy]

Prom operator can be used to configure metrics collection with Prometheus. Although OSM implements the configuration with osm metrics enable command, prom operator is becoming a popular approach to do a better job than OSM CLI tool.

A documentation should cover:

• Installation of Prom Operator on a existing service mesh
• Using the demo application as an example, provide instructions on monitoring OSM control plane and data plane

[allenlsy]

@trstringer @steeling I want some comments about whether we should provide this integration guide. The Prometheus operator official documentation is clear enough about installation in k8s cluster and enable monitoring on user services. Users only need to change the service names following their guides. I think our documentation will be almost a repetition of theirs and we also need to keep our guide updated.