add support for Accept:application/json to /api/v2/metrics

see: https://github.com/ansible/awx/issues/4144

awx/api/views/metrics.py

@@ -31,9 +31,10 @@ class MetricsView(APIView):
     swagger_topic = 'Metrics'
 
     renderer_classes = [renderers.PlainTextRenderer,
+                        renderers.PrometheusJSONRenderer,
                         renderers.BrowsableAPIRenderer,]
 
-    def get(self, request, format='txt'):
+    def get(self, request):
         ''' Show Metrics Details '''
         if (request.user.is_superuser or request.user.is_system_auditor):
             return Response(metrics().decode('UTF-8'))