core: replace GetMiddleware (#885)

* 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

middleware/proxy/setup.go

@@ -20,7 +20,7 @@ func setup(c *caddy.Controller) error {
 		return middleware.Error("proxy", err)
 	}
 
-	t := dnsserver.GetMiddleware(c, "trace")
+	t := dnsserver.GetConfig(c).GetHandler("trace")
 	P := &Proxy{Trace: t}
 	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
 		P.Next = next