PromQL expression to compute difference in a counter but just 1 value for the full Range Time

Hello everybody.

I would like to do a simple query to get just 1 number for each serie, the original metric is a counter, the result should be the last value minus the first value when the last value is greater than the fist (like the incresase funcion does ) . But I need only 1 value por serie.

my_metric[end_time] - my_metric[start_time] if (end_value > start-time)

Which PromQL expression can give this value per serie?

how can I do it with grafana ?