google.protobuf.field_mask_pb2.FieldMask
A list of attributes of the case object that should be
updated as part of this request. Supported values are
priority, display_name, and
subscriber_email_addresses. If no fields are specified,
all supported fields are updated.
WARNING: If you do not provide a field mask, then you might
accidentally clear some fields. For example, if you leave
the field mask empty and do not provide a value for
subscriber_email_addresses, then
subscriber_email_addresses is updated to empty.