Fix panic updating metrics when not enabled

msgbus.go

@@ -600,7 +600,9 @@ func (mb *MessageBus) Subscribe(id, topic string, opts ...SubscribeOption) chan
 				case ch <- msg:
 					n++
 				default:
-					mb.metrics.Counter("bus", "dropped").Inc()
+					if mb.metrics != nil {
+						mb.metrics.Counter("bus", "dropped").Inc()
+					}
 					return ErrBufferFull
 				}
 			}