Enum describing under which condition query expansion should
occur.
Values:
CONDITION_UNSPECIFIED (0):
Unspecified query expansion condition. In this case, server
behavior defaults to
Condition.DISABLED.
DISABLED (1):
Disabled query expansion. Only the exact search query is
used, even if
SearchResponse.total_size
is zero.
AUTO (2):
Automatic query expansion built by the Search
API.
