Custom sensor based on counter to update utility meter

Hello

I order to better calculate my pellet consumtion, I set up a counter that increment when I add a new bag of pellet.

I’d like to sum this up by month with an utility meter (and multiply it by the bag price) so I can have my consumption in € each month.

Problem, how do I create a sensor based on this counter, as I don’t have any device class available ?
and my sensor should be numeric (so the enum class do not work for me I think)

Thx a lot