[infra] Change the name of the num_chunks attribute sent to pubsub.

The results were sent to pubsub with a num_messages attribute, but
the receiver was expecting it to be called num_chunks.

Change-Id: I924386137006367773bcace91578e3dfedd3763c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127881
Reviewed-by: Alexander Thomas <athom@google.com>

tools/bots/post_results_to_pubsub.dart

@@ -64,7 +64,7 @@ main(List<String> args) async {
     message.write(']');
     numMessages++;
     final attributes = {
-      if (line == lines.length) 'num_messages': numMessages.toString()
+      if (line == lines.length) 'num_chunks': numMessages.toString()
     };
     var base64data = base64Encode(utf8.encode(message.toString()));
     var messageObject = {