Values:
READ_REPLICAS_MODE_UNSPECIFIED (0):
If not set, Memorystore Redis backend will default to
READ_REPLICAS_DISABLED.
READ_REPLICAS_DISABLED (1):
If disabled, read endpoint will not be
provided and the instance cannot scale up or
down the number of replicas.
READ_REPLICAS_ENABLED (2):
If enabled, read endpoint will be provided
and the instance can scale up and down the
number of replicas. Not valid for basic tier.