Returns the cost in USD of the AI calls made for each customer, per time bucket, over the date range. The bucket size grows with the length of the range. The default is the sum; use metricType for the average, maximum and so on. Use customers to keep only some customers and costSource to choose which cost to count. For one figure per group over the whole period, a far smaller answer over a long date range, use get_customer_cost_aggregated, which does not take costSource, customers.