Add caveat about custom-metrics API

[abrennan89]

Doc requirement from Markus

Registering the custom-metrics API can clash if the user already has a custom-metric API implementation hooked up in their cluster.

We'll need to document that caveat and document that one will not be able to use custom-metrics (and HPA on concurrency metrics for that matter) on those clusters for now.

Additional context

We'll need to document a caveat, basically saying that if you already have a custom-metrics API implementation in your cluster (TODO: provide command to find that out) you cannot use this and you can deactivate the installation by (TODO: provide command to do that).

[abrennan89]

/assign abrennan89

[abrennan89]

/kind serving

[abrennan89]

/kind needs-eng-input

[abrennan89]

/status in-progress

[abrennan89]

Have a draft ready for this, reviewing with Markus and need to add the commands and then it should be good to go.