middleware/cache: Add metrics (#132)

Add prometheus metrics to the cache handler. This just used prometheus,
if the metrics middleware does not setup the handler, there is nobody
reading these metrics, but they are still reported. Seems the simplest
solution while keeping the whole middleware separation in tact.

middleware/middleware.go

@@ -54,6 +54,8 @@ func (f HandlerFunc) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 	return f(ctx, w, r)
 }
 
+const Namespace = "coredns"
+
 // IndexFile looks for a file in /root/fpath/indexFile for each string
 // in indexFiles. If an index file is found, it returns the root-relative
 // path to the file and true. If no index file is found, empty string