ThresholdRule contains a definition of a threshold which triggers an
alert (a notification of a threshold being crossed) to be sent when
spend goes above the specified amount. Alerts are automatically e-mailed
to users with the Billing Account Administrator role or the Billing
Account User role. The thresholds here have no effect on notifications
sent to anything configured under Budget.all_updates_rule.
Optional. The type of basis used to determine if spend has
passed the threshold. Behavior defaults to CURRENT_SPEND if
not set.