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

core/dnsserver/config.go

@@ -38,6 +38,11 @@ type Config struct {
 
 	// Compiled middleware stack.
 	middlewareChain middleware.Handler
+
+	// Middleware interested in announcing that they exist, so other middleware can call methods
+	// on them should register themselves here. The name should be the name as return by the
+	// Handler's Name method.
+	Registry map[string]middleware.Handler
 }
 
 // GetConfig gets the Config that corresponds to c.
@@ -53,18 +58,3 @@ func GetConfig(c *caddy.Controller) *Config {
 	ctx.saveConfig(c.Key, &Config{})
 	return GetConfig(c)
 }
-
-// GetMiddleware returns the middleware handler that has been added to the config under name.
-// This is useful to inspect if a certain middleware is active in this server.
-// Note that this is order dependent and the order is defined in directives.go, i.e. if your middleware
-// comes before the middleware you are checking; it will not be there (yet).
-func GetMiddleware(c *caddy.Controller, name string) middleware.Handler {
-	conf := GetConfig(c)
-	for _, h := range conf.Middleware {
-		x := h(nil)
-		if name == x.Name() {
-			return x
-		}
-	}
-	return nil
-}