Create a LogRetriever from a client and lambda arn.

        :type client: botocore.client.Logs
        :param client: A ``logs`` client.

        :type lambda_arn: str
        :param lambda_arn: The ARN of the lambda function.

        :return: An instance of ``LogRetriever``.

def create_from_lambda_arn(cls, client, lambda_arn):
        # type: (TypedAWSClient, str) -> LogRetriever
        """Create a LogRetriever from a client and lambda arn.

        :type client: botocore.client.Logs
        :param client: A ``logs`` client.

        :type lambda_arn: str
        :param lambda_arn: The ARN of the lambda function.

        :return: An instance of ``LogRetriever``.

        """
        lambda_name = lambda_arn.split(':')[6]
        log_group_name = '/aws/lambda/%s' % lambda_name
        return cls(client, log_group_name)