The type of basis used to determine if spend has passed the
threshold.
Values:
BASIS_UNSPECIFIED (0):
Unspecified threshold basis.
CURRENT_SPEND (1):
Use current spend as the basis for comparison
against the threshold.
FORECASTED_SPEND (2):
Use forecasted spend for the period as the basis for
comparison against the threshold. FORECASTED_SPEND can only
be set when the budget's time period is a
Filter.calendar_period.
It cannot be set in combination with
Filter.custom_period.