middleware/metrics: fix crash on startup (#318)

Make the methods that handle Metrics all use pointer receivers to fix
sync.Once not being initialized.

Finish the setup_test to test for failures. And make the check for the
address more strict and return an error when it does not have a port
number.

Add a toplevel test that starts a CoreDNS server with metrics enabled
so we catch these errors in the future.

middleware/metrics/handler.go

@@ -13,7 +13,7 @@ import (
 )
 
 // ServeDNS implements the Handler interface.
-func (m Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	state := request.Request{W: w, Req: r}
 
 	qname := state.QName()