Fix metrics representation in TenancySerializer (#452)

This was mistakenly updated during code formatting to return a tuple
with a single element, rather than a bare string. This caused the UI to
panic as it was trying to apply string functions to a list object.

Restore the original string representation to fix the UI panic.

Author: m-bull

api/azimuth/serializers.py

@@ -244,7 +244,7 @@ def to_representation(self, obj):
                     "metrics": (
                         cloud_settings.METRICS.TENANT_METRICS_URL_TEMPLATE.format(
                             tenant_id=obj.id
-                        ),
+                        )
                     )
                 }
             )