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.
