Values:
STATE_UNSPECIFIED (0):
The default value. This value is used if the
state is unknown.
CREATING (1):
The preview is being created.
SUCCEEDED (2):
The preview has succeeded.
APPLYING (3):
The preview is being applied.
STALE (4):
The preview is stale. A preview can become
stale if a revision has been applied after this
preview was created.
DELETING (5):
The preview is being deleted.
FAILED (6):
The preview has encountered an unexpected
error.
DELETED (7):
The preview has been deleted.
