* core: replace GetMiddleware
See the discussion in #881. GetMiddleware would add a `nil` middleware
to the callstack thereby breaking functionality.
This PR drops it in favor of RegisterHandler which is a completely
standalone registry for middleware that want to let it self know to
other middleware.
Currenly *autopath* uses this to call *kubernetes*'s AutoPath method
for dynamic autopathing.
* Drop GetMiddleware
* Register metrics
* drop the panic
