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, 69 insertions, 0 deletions

diff --git a/tests/Unit/Controllers/Metrics/MetricsEngineTest.php b/tests/Unit/Controllers/Metrics/MetricsEngineTest.php
new file mode 100644
index 00000000..b810b10a
--- /dev/null
+++ b/tests/Unit/Controllers/Metrics/MetricsEngineTest.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Engelsystem\Test\Unit\Controllers\Metrics;
+
+use Engelsystem\Controllers\Metrics\MetricsEngine;
+use Engelsystem\Test\Unit\TestCase;
+
+class MetricsEngineTest extends TestCase
+{
+    /**
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::get
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::formatData
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::renderLabels
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::renderValue
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::formatValue
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::escape
+     */
+    public function testGet()
+    {
+        $engine = new MetricsEngine();
+
+        $this->assertEquals('', $engine->get('/metrics'));
+
+        $this->assertEquals('engelsystem_users 13', $engine->get('/metrics', ['users' => 13]));
+
+        $this->assertEquals('engelsystem_bool_val 0', $engine->get('/metrics', ['bool_val' => false]));
+
+        $this->assertEquals('# Lorem \n Ipsum', $engine->get('/metrics', ["Lorem \n Ipsum"]));
+
+        $this->assertEquals(
+            'engelsystem_foo{lorem="ip\\\\sum"} \\"lorem\\n\\\\ipsum\\"',
+            $engine->get('/metrics', [
+                'foo' => ['labels' => ['lorem' => 'ip\\sum'], 'value' => "\"lorem\n\\ipsum\""]
+            ])
+        );
+
+        $this->assertEquals(
+            'engelsystem_foo_count{bar="14"} 42',
+            $engine->get('/metrics', ['foo_count' => ['labels' => ['bar' => 14], 'value' => 42],])
+        );
+
+        $this->assertEquals(
+            'engelsystem_lorem{test="123"} NaN' . "\n" . 'engelsystem_lorem{test="456"} 999.99',
+            $engine->get('/metrics', [
+                'lorem' => [
+                    ['labels' => ['test' => 123], 'value' => 'NaN'],
+                    ['labels' => ['test' => 456], 'value' => 999.99],
+                ],
+            ])
+        );
+
+        $this->assertEquals(
+            "# HELP engelsystem_test Some help\\n  text\n# TYPE engelsystem_test counter\nengelsystem_test 99",
+            $engine->get('/metrics', ['test' => ['help' => "Some help\n  text", 'type' => 'counter', 'value' => 99]])
+        );
+    }
+
+    /**
+     * @covers \Engelsystem\Controllers\Metrics\MetricsEngine::canRender
+     */
+    public function testCanRender()
+    {
+        $engine = new MetricsEngine();
+
+        $this->assertFalse($engine->canRender('/'));
+        $this->assertFalse($engine->canRender('/metrics.foo'));
+        $this->assertTrue($engine->canRender('/metrics'));
+    }
+}