Missing note in Prometheus metrics section

[Prometheus metrics policy](https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.6/html/administering_the_api_gateway/apicast_policies#prometheus-metrics) makes no mention of how to get access to the additional metrics service_id & service_system_name so it is not clear to the user how they should do so.

This is achieved by using the environment variable APICAST_EXTENDED_METRICS.

Additionally the APICAST_EXTENDED_METRICS environment variable is not indexed in the [apicast environment variables](https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.6/html/administering_the_api_gateway/apicast_environment_variables) section.