Added tests and countable interface

Closes #3445425

core/lib/Drupal/Core/Template/Attribute.php

@@ -67,7 +67,7 @@
  * @see \Drupal\Component\Render\PlainTextOutput::renderFromHtml()
  * @see \Drupal\Component\Utility\UrlHelper::stripDangerousProtocols()
  */
-class Attribute implements \ArrayAccess, \IteratorAggregate, MarkupInterface {
+class Attribute implements \ArrayAccess, \IteratorAggregate, \Countable, MarkupInterface {
 
   /**
    * Stores the attribute data.
@@ -382,6 +382,16 @@ public function jsonSerialize(): string {
     return (string) $this;
   }
 
+  /**
+   * Returns the number of attributes stored in the Attribute object.
+   *
+   * @return int
+   *   The number of attributes.
+   */
+  public function count(): int {
+    return count($this->storage);
+  }
+
   /**
    * Merges an Attribute object into the current storage.
    *