Issue #3205503: Create a session count metric

src/Plugin/MetricsCollector/SessionCountCollector.php

+<?php
+
+namespace Drupal\prometheus_exporter\Plugin\MetricsCollector;
+
+use Drupal\Core\Database\Connection;
+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 sessions.
+ *
+ * @MetricsCollector(
+ *   id = "session_count",
+ *   title = @Translation("Session count"),
+ *   description = @Translation("Provides metrics for the number of sessions.")
+ * )
+ */
+class SessionCountCollector extends BaseMetricsCollector implements ContainerFactoryPluginInterface {
+
+  /**
+   * The PDO database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $connection;
+
+  /**
+   * SessionCountCollector 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 $userType => $count) {
+      $gauge->set($count, ['user' => $userType]);
+    }
+    $metrics[] = $gauge;
+    return $metrics;
+  }
+
+  /**
+   * Query the session table to retrieve the number of sessions.
+   *
+   * @return array
+   *   Count of sessions for anonymous and authenticated users.
+   */
+  protected function query() {
+    return [
+      'anonymous' => (int) $this->connection->query('SELECT COUNT(1) FROM sessions WHERE uid = :anonymous', [':anonymous' => 0])->fetchField(),
+      'authenticated' => (int) $this->connection->query('SELECT COUNT(1) FROM sessions WHERE uid > :anonymous', [':anonymous' => 0])->fetchField(),
+    ];
+  }
+
+}