JSON Datasource and Variables not expanding

Having taken the simple JSON datasource as a starting point, I have it working against an Informix database. The query below works fine and returns as expected.

{“table”: “oni_perf_v”, “key”: {“name”: “metric”, “value”: “br”}, “col_ts”: “ts”, “col_value”: “value0”}

I have a Metric variable that appears to be working and I can see on the dashboard, but the following query fails, the console log shows [[Metric]] being passed and not expanded to the pulldown value as I believe it should be

{“table”: “oni_perf_v”, “key”: {“name”: “metric”, “value”: “[[Metric]]”}, “col_ts”: “ts”, “col_value”: “value0”}

As I am new to Grafana and js/angular, I am sure the issue is me but I don’t understand what I have missed