Reduce possibility of breaking the symfony Toolbar

By only dumping the needed relevant information needed for the toolbar/debug page, we avoid dumping data which could break the symfony toolbar (FileProfilerStorage), which cannot serialize Closures. This can happen if the publisher uses a factory
In this case the UsageTrackingTokenStorage of the symfony authentication component uses the ContainerInterface/SessionLocator, which creates a Closure
Fixes #19

Author: lunetics

src/DataCollector/MercureDataCollector.php

@@ -36,9 +36,13 @@ public function collect(Request $request, Response $response, \Throwable $except
             'count' => 0,
             'duration' => 0.0,
             'memory' => 0,
-            'publishers' => iterator_to_array($this->publishers),
+            'publishers' => [],
         ];
 
+        foreach ($this->publishers as $name => $publisher) {
+            $this->data['publishers'][$name]['messages'] = $publisher->getMessages();
+        }
+
         foreach ($this->publishers as $name => $publisher) {
             $this->data['duration'] += $publisher->getDuration();
             $this->data['memory'] += $publisher->getMemory();