Implement reset() method in StatsdDataCollector

After upgrade to Symfony 3.4, there is a deprecation error caused by this class:

> Implementing "Symfony\Component\HttpKernel\DataCollector\DataCollectorInterface" without the "reset()" method is deprecated since Symfony 3.4 and will be unsupported in 4.0 for class "M6Web\Bundle\StatsdBundle\DataCollector\StatsdDataCollector"

This PR introduces what the deprecation notice asks for.

src/DataCollector/StatsdDataCollector.php

@@ -18,12 +18,20 @@ class StatsdDataCollector extends DataCollector
      * Construct the data collector
      */
     public function __construct()
+    {
+        $this->reset();
+    }
+
+    /**
+     * Reset the data collector to initial state
+     */
+    public function reset()
     {
         $this->statsdClients      = [];
         $this->data['clients']    = [];
         $this->data['operations'] = 0;
     }
-
+    
     /**
      * Kernel event
      *