Values:
NO_CHANGE (0):
No change has occurred. Used only to send an updated
resume_token.
ADD (1):
The targets have been added.
REMOVE (2):
The targets have been removed.
CURRENT (3):
The targets reflect all changes committed before the targets
were added to the stream.
This will be sent after or with a `read_time` that is
greater than or equal to the time at which the targets were
added.
Listeners can wait for this change if read-after-write
semantics are desired.
RESET (4):
The targets have been reset, and a new initial state for the
targets will be returned in subsequent changes.
After the initial state is complete, `CURRENT` will be
returned even if the target was previously indicated to be
`CURRENT`.