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/metrics/setup.go

@@ -38,6 +38,9 @@ func setup(c *caddy.Controller) error {
 	}
 	c.OnFinalShutdown(m.OnShutdown)
 
+	// Also register metrics for use in other middleware.
+	dnsserver.GetConfig(c).RegisterHandler(m)
+
 	return nil
 }