def list_instances(self, filter_="", page_size=None, page_token=None):
        """List instances for the client's project.

        See
        https://cloud.google.com/spanner/reference/rpc/google.spanner.admin.database.v1#google.spanner.admin.database.v1.InstanceAdmin.ListInstances

        :type filter_: string
        :param filter_: (Optional) Filter to select instances listed.  See
                        the ``ListInstancesRequest`` docs above for examples.

        :type page_size: int
        :param page_size:
            Optional. The maximum number of instances in each page of results
            from this request. Non-positive values are ignored. Defaults
            to a sensible value set by the API.

        :type page_token: str
        :param page_token:
            Optional. If present, return the next batch of instances, using
            the value, which must correspond to the ``nextPageToken`` value
            returned in the previous response.  Deprecated: use the ``pages``
            property of the returned iterator instead of manually passing
            the token.

        :rtype: :class:`~google.api_core.page_iterator.Iterator`
        :returns:
            Iterator of :class:`~google.cloud.spanner_v1.instance.Instance`
            resources within the client's project.
        """
        metadata = _metadata_with_prefix(self.project_name)
        path = "projects/%s" % (self.project,)
        page_iter = self.instance_admin_api.list_instances(
            path, page_size=page_size, metadata=metadata
        )
        page_iter.item_to_value = self._item_to_instance
        page_iter.next_page_token = page_token
        return page_iter