[extension/zpages] Implement rpcz page or delete mentions

[atoulme]

Is your feature request related to a problem? Please describe.
The zpages extension README mentions /debug/rpcz page:

The Rpcz route is available to help examine statistics of remote procedure calls (RPCs) 
that are properly instrumented. For example when using gRPC

However, this page is not implemented.

Describe the solution you'd like
Remove the rpcz mention or draft a plan to implement it.

Describe alternatives you've considered
I have done a bit of search in git history but I have not found a mention of rcpz being implemented before, there's no mention in issues or pull requests either.