Fix prometheus handlers (#4311)

* fix prometheus handlers not serving responses

* add basic integration test to prometheus handler

* modify .gitignore to ignore all ffmpeg versioned folders instead of just any file exactly matching "ffmpeg"

test/automated/api/006_integrations.test.js

@@ -172,3 +172,8 @@ test('send an external integration action using access token expecting failure',
 		.send(payload)
 		.expect(401);
 });
+
+test('test prometheus metrics', async() => {
+	const res = await getAdminResponse('prometheus');
+	expect(res.text).toMatch(/owncast_instance_cpu_usage/);
+});