Draft: Start counting and checking for cache tag lookup queries.

Closes #3436146

core/lib/Drupal/Core/Cache/Event/CommonCacheTagsEvent.php

+<?php
+
+namespace Drupal\Core\Cache\Event;
+
+use Drupal\Component\EventDispatcher\Event;
+
+/**
+ * Represents a "common cache tags" event.
+ */
+class CommonCacheTagsEvent extends Event {
+
+  /**
+   * The common cache tags.
+   */
+  protected array $tags = [];
+
+  /**
+   * Adds cache tags to the list of common cache tags.
+   *
+   * @param array $tags
+   *   The cache tags to add.
+   */
+  public function addCacheTags(array $tags): void {
+    $this->tags = array_unique(array_merge($this->tags, $tags));
+  }
+
+  /**
+   * Retrieves the list of common cache tags.
+   *
+   * @return string[]
+   *   The common cache tags.
+   */
+  public function getCacheTags(): array {
+    return $this->tags;
+  }
+
+}