Error when watching service endpoints across all namespaces

This was a bug reported by @bketelsen who is using the proxy/config/api source channel.

A watch of service endpoints across all namespaces was throwing an error because it was using the older key method that was missed in an update of the enforce namespace PR.
Nit: get the namespace out of the context before calling the key construction function; I can't think of a reason to pass the whole context.