Values:
PARTITION_KEY_UNSPECIFIED (0):
Unspecified partition key. If used, it means
using non-partitioned table.
READ_TIME (1):
The time when the snapshot is taken. If specified as
partition key, the result table(s) is partitoned by the
additional timestamp column, readTime. If [read_time] in
ExportAssetsRequest is specified, the readTime column's
value will be the same as it. Otherwise, its value will be
the current time that is used to take the snapshot.
REQUEST_TIME (2):
The time when the request is received and
started to be processed. If specified as
partition key, the result table(s) is partitoned
by the requestTime column, an additional
timestamp column representing when the request
was received.