Make sure contract provided capacity (in billable units) is deducted from tally in billable units

Measured units are what are captured during tally.

Billable units are measured units with the billing_factor applied.

For example, given a contract with 1 4vCPU capacity, and a tally measurement of 4 vCPUs, the contract should cover the entire tally measurement (assuming it's the first measurement covered by the contract).