fix a few issues with license counts in /api/v2/metrics/

- switched these to gauges so people can track them over time
- fixed a typo that caused `free_instances` to always be zero

awx/main/tests/functional/analytics/test_metrics.py

@@ -28,6 +28,8 @@ EXPECTED_VALUES = {
     'awx_instance_cpu':0.0,
     'awx_instance_memory':0.0,
     'awx_instance_info':1.0,
+    'awx_license_instance_total':0,
+    'awx_license_instance_free':0,
 }