Issue #3205502: Add a flood table record metric

src/Plugin/MetricsCollector/FloodCountCollector.php

+<?php
+
+namespace Drupal\prometheus_exporter\Plugin\MetricsCollector;
+
+use Drupal\Core\Database\Connection;
+use Drupal\Core\Flood\DatabaseBackend;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\prometheus_exporter\Plugin\BaseMetricsCollector;
+use PNX\Prometheus\Gauge;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Collects metrics for the number of Flood entries.
+ *
+ * @MetricsCollector(
+ *   id = "flood_count",
+ *   title = @Translation("Flood count"),
+ *   description = @Translation("Provides metrics for the number of Flood entries.")
+ * )
+ */
+class FloodCountCollector extends BaseMetricsCollector implements ContainerFactoryPluginInterface {
+
+  /**
+   * The PDO database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $connection;
+
+  /**
+   * FloodCountCollector constructor.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin_id for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   The PDO database connection.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, Connection $connection) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->connection = $connection;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('database')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function collectMetrics() {
+    $gauge = new Gauge($this->getNamespace(), 'total', $this->getDescription());
+    foreach ($this->query() as $event => $count) {
+      $gauge->set($count, ['event' => $event]);
+    }
+    $metrics[] = $gauge;
+    return $metrics;
+  }
+
+  /**
+   * Query the number of records in the flood table.
+   *
+   * @return array
+   *   Associative array of the number of events, keyed by the event type.
+   */
+  protected function query() {
+    // The flood table is generated on-demand and may not exist.
+    if (!$this->connection->schema()->tableExists(DatabaseBackend::TABLE_NAME)) {
+      return [];
+    }
+
+    $query = $this->connection->select(DatabaseBackend::TABLE_NAME, 'flood')
+      ->fields('flood', ['event'])
+      ->groupBy('event');
+    $query->addExpression('count(fid)', 'count');
+
+    return $query
+      ->execute()
+      ->fetchAllKeyed(0, 1);
+  }
+
+}