Document the new metrics proxy changes

[nyechiel]

With submariner-io/submariner-operator#2182 merged, users no longer need to open ports 8080 and 8081 on hosts for querying metrics. Users should continue to query existing submariner-gateway-metrics and submariner-globalnet-metrics services for exported metrics.

TO DO:

• Update https://submariner.io/operations/monitoring/ with explanation of the new metrics proxy component and what it does
• https://submariner.io/operations/deployment/subctl/ has references to --metrics-ports. Need to update the default values which are now 32780 and 32781.
• Update the metrics part under https://submariner.io/getting-started/#prerequisites