Implemented /metrics endpoint and reimplemented /stats

closes #418 (/metrics endpoint)

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

1 files changed, 74 insertions, 0 deletions

diff --git a/tests/Unit/Controllers/Metrics/StatsTest.php b/tests/Unit/Controllers/Metrics/StatsTest.php
new file mode 100644
index 00000000..1618b99b
--- /dev/null
+++ b/tests/Unit/Controllers/Metrics/StatsTest.php
@@ -0,0 +1,74 @@
+<?php
+
+namespace Engelsystem\Test\Unit\Controllers\Metrics;
+
+use Engelsystem\Controllers\Metrics\Stats;
+use Engelsystem\Models\User\State;
+use Engelsystem\Models\User\User;
+use Engelsystem\Test\Unit\HasDatabase;
+use Engelsystem\Test\Unit\TestCase;
+use Illuminate\Support\Str;
+
+class StatsTest extends TestCase
+{
+    use HasDatabase;
+
+    /**
+     * @covers \Engelsystem\Controllers\Metrics\Stats::newUsers
+     * @covers \Engelsystem\Controllers\Metrics\Stats::getQuery
+     * @covers \Engelsystem\Controllers\Metrics\Stats::__construct
+     */
+    public function testNewUsers()
+    {
+        $this->initDatabase();
+        $this->addUsers();
+
+        $stats = new Stats($this->database);
+        $this->assertEquals(2, $stats->newUsers());
+    }
+
+    /**
+     * @covers \Engelsystem\Controllers\Metrics\Stats::arrivedUsers
+     */
+    public function testArrivedUsers()
+    {
+        $this->initDatabase();
+        $this->addUsers();
+
+        $stats = new Stats($this->database);
+        $this->assertEquals(3, $stats->arrivedUsers());
+    }
+
+    /**
+     * Add some example users
+     */
+    protected function addUsers()
+    {
+        $this->addUser();
+        $this->addUser();
+        $this->addUser(['arrived' => 1]);
+        $this->addUser(['arrived' => 1, 'active' => 1]);
+        $this->addUser(['arrived' => 1, 'active' => 1]);
+    }
+
+    /**
+     * @param array $state
+     */
+    protected function addUser(array $state = [])
+    {
+        $name = 'user_' . Str::random(5);
+
+        $user = new User([
+            'name'     => $name,
+            'password' => '',
+            'email'    => $name . '@engel.example.com',
+            'api_key'  => '',
+        ]);
+        $user->save();
+
+        $state = new State($state);
+        $state->user()
+            ->associate($user)
+            ->save();
+    }
+}