The attribute(or function) for which the custom ranking is to
be applied.
Values:
ATTRIBUTE_TYPE_UNSPECIFIED (0):
Unspecified AttributeType.
NUMERICAL (1):
The value of the numerical field will be used to dynamically
update the boost amount. In this case, the attribute_value
(the x value) of the control point will be the actual value
of the numerical field for which the boost_amount is
specified.
FRESHNESS (2):
For the freshness use case the attribute value will be the
duration between the current time and the date in the
datetime field specified. The value must be formatted as an
XSD dayTimeDuration value (a restricted subset of an ISO
8601 duration value). The pattern for this is:
[nD][TnH][nM][nS]]. E.g. 5D, 3DT12H30M,
T24H.
