How to identify if metric X has dropped by 10% in the last 5 minutes

I am trying to configure alerts through which I can alert if metric X has dropped by 10% in the last 5 minutes and identify the corresponding time series

sum(metricname{_namespace_="metrics"}) / sum(metricname{_namespace_="metrics"} offset 5m) < 0.90

How can i do grouping in above Prom QL for a specific dimension