Find usage on API Keys.
        Update `self.limits`.

def _find_usage_api_keys(self):
        """
        Find usage on API Keys.
        Update `self.limits`.
        """
        logger.debug('Finding usage for API Keys')
        key_count = 0
        paginator = self.conn.get_paginator('get_api_keys')
        for resp in paginator.paginate():
            key_count += len(resp['items'])
        self.limits['API keys per account']._add_current_usage(
            key_count, aws_type='AWS::ApiGateway::ApiKey'
        )