util/eventbus: initial debugging facilities for the event bus

Enables monitoring events as they flow, listing bus clients, and
snapshotting internal queues to troubleshoot stalls.

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

util/eventbus/publish.go

@@ -52,7 +52,7 @@ func (p *Publisher[T]) Publish(v T) {
 	default:
 	}
 
-	evt := publishedEvent{
+	evt := PublishedEvent{
 		Event: v,
 		From:  p.client,
 	}