Which type of metric is best suited for a graph of the number of orders per second?

The best metric for a graph of the number of orders per second is a rate metric. It shows how many events (orders) occur per unit of time, for example, per second. This type of metric allows tracking dynamics and peak loads in real time.

Example: if you have a counter of the total number of orders, you can calculate the rate using the rate function in monitoring systems (like Prometheus, etc.) to get the number of orders per second.