Don't expose certificate metrics for default server

The default server has a self signed certificate so it's not important to monitor

core/pkg/ingress/controller/metrics.go

@@ -75,7 +75,9 @@ func incReloadErrorCount() {
 func setSSLExpireTime(servers []*ingress.Server) {
 
 	for _, s := range servers {
+		if s.Hostname != defServerName {
 			sslExpireTime.WithLabelValues(s.Hostname).Set(float64(s.SSLExpireTime.Unix()))
 		}
+	}
 
 }