Implemented /metrics endpoint and reimplemented /stats

closes #418 (/metrics endpoint)

Usage:
```yaml
scrape_configs:
  - job_name: 'engelsystem'
    static_configs:
    - targets: ['engelsystem.example.com:80']
```

4 files changed, 412 insertions, 5 deletions

diff --git a/src/Controllers/Metrics/Controller.php b/src/Controllers/Metrics/Controller.php
new file mode 100644
index 00000000..01fe1d6a
--- /dev/null
+++ b/src/Controllers/Metrics/Controller.php
@@ -0,0 +1,131 @@
+<?php
+
+namespace Engelsystem\Controllers\Metrics;
+
+use Engelsystem\Config\Config;
+use Engelsystem\Controllers\BaseController;
+use Engelsystem\Http\Exceptions\HttpForbidden;
+use Engelsystem\Http\Request;
+use Engelsystem\Http\Response;
+
+class Controller extends BaseController
+{
+    /** @var Config */
+    protected $config;
+
+    /** @var MetricsEngine */
+    protected $engine;
+
+    /** @var Request */
+    protected $request;
+
+    /** @var Response */
+    protected $response;
+
+    /** @var Stats */
+    protected $stats;
+
+    /**
+     * @param Response      $response
+     * @param MetricsEngine $engine
+     * @param Config        $config
+     * @param Request       $request
+     * @param Stats         $stats
+     */
+    public function __construct(
+        Response $response,
+        MetricsEngine $engine,
+        Config $config,
+        Request $request,
+        Stats $stats
+    ) {
+        $this->config = $config;
+        $this->engine = $engine;
+        $this->request = $request;
+        $this->response = $response;
+        $this->stats = $stats;
+    }
+
+    /**
+     * @return Response
+     */
+    public function metrics()
+    {
+        $now = microtime(true);
+        $this->checkAuth();
+
+        $data = [
+            $this->config->get('app_name') . ' stats',
+            'users'                => [
+                'type' => 'gauge',
+                ['labels' => ['state' => 'incoming'], 'value' => $this->stats->newUsers()],
+                ['labels' => ['state' => 'arrived', 'working' => 'no'], 'value' => $this->stats->arrivedUsers(false)],
+                ['labels' => ['state' => 'arrived', 'working' => 'yes'], 'value' => $this->stats->arrivedUsers(true)],
+            ],
+            'users_working'        => [
+                'type' => 'gauge',
+                ['labels' => ['freeloader' => false], $this->stats->currentlyWorkingUsers(false)],
+                ['labels' => ['freeloader' => true], $this->stats->currentlyWorkingUsers(true)],
+            ],
+            'work_seconds'         => [
+                'type' => 'gauge',
+                ['labels' => ['state' => 'done'], 'value' => $this->stats->workSeconds(true, false)],
+                ['labels' => ['state' => 'planned'], 'value' => $this->stats->workSeconds(false, false)],
+                ['labels' => ['state' => 'freeloaded'], 'value' => $this->stats->workSeconds(null, true)],
+            ],
+            'registration_enabled' => ['type' => 'gauge', $this->config->get('registration_enabled')],
+        ];
+
+        $data['scrape_duration_seconds'] = [
+            'type' => 'gauge',
+            'help' => 'Duration of the current request',
+            microtime(true) - $this->request->server->get('REQUEST_TIME_FLOAT', $now)
+        ];
+
+        return $this->response
+            ->withHeader('Content-Type', 'text/plain; version=0.0.4')
+            ->withContent($this->engine->get('/metrics', $data));
+    }
+
+    /**
+     * @return Response
+     */
+    public function stats()
+    {
+        $this->checkAuth(true);
+
+        $data = [
+            'user_count'         => $this->stats->newUsers() + $this->stats->arrivedUsers(),
+            'arrived_user_count' => $this->stats->arrivedUsers(),
+            'done_work_hours'    => round($this->stats->workSeconds(true) / 60 / 60, 0),
+            'users_in_action'    => $this->stats->currentlyWorkingUsers(),
+        ];
+
+        return $this->response
+            ->withHeader('Content-Type', 'application/json')
+            ->withContent(json_encode($data));
+    }
+
+    /**
+     * Ensure that the if the request is authorized
+     *
+     * @param bool $isJson
+     */
+    protected function checkAuth($isJson = false)
+    {
+        $apiKey = $this->config->get('api_key');
+        if (empty($apiKey) || $this->request->get('api_key') == $apiKey) {
+            return;
+        }
+
+        $message = 'The api_key is invalid';
+        $headers = [];
+
+        if ($isJson) {
+            $message = json_encode(['error' => $message]);
+            $headers['Content-Type'] = 'application/json';
+        }
+
+        throw new HttpForbidden($message, $headers);
+    }
+}
diff --git a/src/Controllers/Metrics/MetricsEngine.php b/src/Controllers/Metrics/MetricsEngine.php
new file mode 100644
index 00000000..eeb47d8a
--- /dev/null
+++ b/src/Controllers/Metrics/MetricsEngine.php
@@ -0,0 +1,137 @@
+<?php
+
+namespace Engelsystem\Controllers\Metrics;
+
+use Engelsystem\Renderer\EngineInterface;
+
+class MetricsEngine implements EngineInterface
+{
+    /**
+     * Render metrics
+     *
+     * @example $data = ['foo' => [['labels' => ['foo'=>'bar'], 'value'=>42]], 'bar'=>123]
+     *
+     * @param string  $path
+     * @param mixed[] $data
+     * @return string
+     */
+    public function get($path, $data = []): string
+    {
+        $return = [];
+        foreach ($data as $name => $list) {
+            if (is_int($name)) {
+                $return[] = '# ' . $this->escape($list);
+                continue;
+            }
+
+            $list = is_array($list) ? $list : [$list];
+            $name = 'engelsystem_' . $name;
+
+            if (isset($list['help'])) {
+                $return[] = sprintf('# HELP %s %s', $name, $this->escape($list['help']));
+                unset($list['help']);
+            }
+
+            if (isset($list['type'])) {
+                $return[] = sprintf('# TYPE %s %s', $name, $list['type']);
+                unset($list['type']);
+            }
+
+            $list = (!isset($list['value']) || !isset($list['labels'])) ? $list : [$list];
+            foreach ($list as $row) {
+                $row = is_array($row) ? $row : [$row];
+
+                $return[] = $this->formatData($name, $row);
+            }
+        }
+
+        return implode("\n", $return);
+    }
+
+    /**
+     * @param string $path
+     * @return bool
+     */
+    public function canRender($path): bool
+    {
+        return $path == '/metrics';
+    }
+
+    /**
+     * @param string      $name
+     * @param array|mixed $row
+     * @see https://prometheus.io/docs/instrumenting/exposition_formats/
+     * @return string
+     */
+    protected function formatData($name, $row): string
+    {
+        return sprintf(
+            '%s%s %s',
+            $name,
+            $this->renderLabels($row),
+            $this->renderValue($row));
+    }
+
+    /**
+     * @param array|mixed $row
+     * @return mixed
+     */
+    protected function renderLabels($row): string
+    {
+        $labels = [];
+        if (!is_array($row) || empty($row['labels'])) {
+            return '';
+        }
+
+        foreach ($row['labels'] as $type => $value) {
+            $labels[$type] = $type . '="' . $this->formatValue($value) . '"';
+        }
+
+        return '{' . implode(',', $labels) . '}';
+    }
+
+    /**
+     * @param array|mixed $row
+     * @return mixed
+     */
+    protected function renderValue($row)
+    {
+        if (isset($row['value'])) {
+            return $this->formatValue($row['value']);
+        }
+
+        return $this->formatValue(array_pop($row));
+    }
+
+    /**
+     * @param mixed $value
+     * @return mixed
+     */
+    protected function formatValue($value)
+    {
+        if (is_bool($value)) {
+            return (int)$value;
+        }
+
+        return $this->escape($value);
+    }
+
+    /**
+     * @param mixed $value
+     * @return mixed
+     */
+    protected function escape($value)
+    {
+        $replace = [
+            '\\' => '\\\\',
+            '"'  => '\\"',
+            "\n" => '\\n',
+        ];
+
+        return str_replace(
+            array_keys($replace),
+            array_values($replace),
+            $value
+        );
+    }
+}
diff --git a/src/Controllers/Metrics/Stats.php b/src/Controllers/Metrics/Stats.php
new file mode 100644
index 00000000..891f8c80
--- /dev/null
+++ b/src/Controllers/Metrics/Stats.php
@@ -0,0 +1,144 @@
+<?php
+
+namespace Engelsystem\Controllers\Metrics;
+
+use Engelsystem\Database\Database;
+use Illuminate\Database\Query\Builder as QueryBuilder;
+use Illuminate\Database\Query\Expression as QueryExpression;
+
+class Stats
+{
+    /** @var Database */
+    protected $db;
+
+    /**
+     * @param Database $db
+     */
+    public function __construct(Database $db)
+    {
+        $this->db = $db;
+    }
+
+    /**
+     * The number of not arrived users
+     *
+     * @param null $working
+     * @return int
+     */
+    public function arrivedUsers($working = null): int
+    {
+        $query = $this
+            ->getQuery('users')
+            ->join('users_state', 'user_id', '=', 'id')
+            ->where('arrived', '=', 1);
+
+        if (!is_null($working)) {
+            // @codeCoverageIgnoreStart
+            $query
+                ->leftJoin('UserWorkLog', 'UserWorkLog.user_id', '=', 'users.id')
+                ->leftJoin('ShiftEntry', 'ShiftEntry.UID', '=', 'users.id')
+                ->groupBy('users.id');
+
+            $query->where(function ($query) use ($working) {
+                /** @var QueryBuilder $query */
+                if ($working) {
+                    $query
+                        ->whereNotNull('ShiftEntry.SID')
+                        ->orWhereNotNull('UserWorkLog.work_hours');
+
+                    return;
+                }
+                $query
+                    ->whereNull('ShiftEntry.SID')
+                    ->whereNull('UserWorkLog.work_hours');
+            });
+            // @codeCoverageIgnoreEnd
+        }
+
+        return $query
+            ->count();
+    }
+
+    /**
+     * The number of not arrived users
+     *
+     * @return int
+     */
+    public function newUsers(): int
+    {
+        return $this
+            ->getQuery('users')
+            ->join('users_state', 'user_id', '=', 'id')
+            ->where('arrived', '=', 0)
+            ->count();
+    }
+
+    /**
+     * The number of currently working users
+     *
+     * @param null $freeloaded
+     * @return int
+     * @codeCoverageIgnore
+     */
+    public function currentlyWorkingUsers($freeloaded = null): int
+    {
+        $query = $this
+            ->getQuery('users')
+            ->join('ShiftEntry', 'ShiftEntry.UID', '=', 'users.id')
+            ->join('Shifts', 'Shifts.SID', '=', 'ShiftEntry.SID')
+            ->where('Shifts.start', '<=', time())
+            ->where('Shifts.end', '>', time());
+
+        if (!is_null($freeloaded)) {
+            $query->where('ShiftEntry.freeloaded', '=', $freeloaded);
+        }
+
+        return $query->count();
+    }
+
+    /**
+     * The number of worked shifts
+     *
+     * @param bool|null $done
+     * @param bool|null $freeloaded
+     * @return int
+     * @codeCoverageIgnore
+     */
+    public function workSeconds($done = null, $freeloaded = null): int
+    {
+        $query = $this
+            ->getQuery('ShiftEntry')
+            ->join('Shifts', 'Shifts.SID', '=', 'ShiftEntry.SID');
+
+        if (!is_null($freeloaded)) {
+            $query->where('freeloaded', '=', $freeloaded);
+        }
+
+        if (!is_null($done)) {
+            $query->where('end', ($done == true ? '<' : '>='), time());
+        }
+
+        return $query->sum($this->raw('end - start'));
+    }
+
+    /**
+     * @param string $table
+     * @return QueryBuilder
+     */
+    protected function getQuery(string $table): QueryBuilder
+    {
+        return $this->db
+            ->getConnection()
+            ->table($table);
+    }
+
+    /**
+     * @param mixed $value
+     * @return QueryExpression
+     * @codeCoverageIgnore
+     */
+    protected function raw($value)
+    {
+        return $this->db->getConnection()->raw($value);
+    }
+}
diff --git a/src/Middleware/LegacyMiddleware.php b/src/Middleware/LegacyMiddleware.php
index b1315fda..8524764f 100644
--- a/src/Middleware/LegacyMiddleware.php
+++ b/src/Middleware/LegacyMiddleware.php
@@ -26,7 +26,6 @@ class LegacyMiddleware implements MiddlewareInterface
         'shift_entries',
         'shifts',
         'shifts_json_export',
-        'stats',
         'users',
         'user_driver_licenses',
         'user_password_recovery',
@@ -122,10 +121,6 @@ class LegacyMiddleware implements MiddlewareInterface
             case 'shifts_json_export':
                 require_once realpath(__DIR__ . '/../../includes/controller/shifts_controller.php');
                 shifts_json_export_controller();
-            /** @noinspection PhpMissingBreakStatementInspection */
-            case 'stats':
-                require_once realpath(__DIR__ . '/../../includes/pages/guest_stats.php');
-                guest_stats();
             case 'user_password_recovery':
                 require_once realpath(__DIR__ . '/../../includes/controller/users_controller.php');
                 $title = user_password_recovery_title();