MMS Public API Metrics should only be visible to OnPrem

Recently, the documentation for the MMS Public API's new Metrics resource was published. At the time, I forgot to mention that it should only be visible in the OnPrem docs. Eventually, we'll be exposing Metrics to MMS Premium customers too, but I'll file a separate ticket when the time comes.

I'm not sure if this page should be totally hidden from the Cloud manual, or if it would be better to add a note at the top of it stating that it's only available in On-Premise MMS. Your call.