fix tsan data race in attributes (grpc#10805)

netty/src/main/java/io/grpc/netty/NettyClientHandler.java

@@ -131,7 +131,7 @@ protected void handleNotInUse() {
 
   private WriteQueue clientWriteQueue;
   private Http2Ping ping;
-  private Attributes attributes;
+  private volatile Attributes attributes;
   private InternalChannelz.Security securityInfo;
   private Status abruptGoAwayStatus;
   private Status channelInactiveReason;