client.go
func OptClientPoolSizePerRoute(size int) ClientOption {
	return func(options *ClientOptions) error {
		options.PoolSizePerRoute = size
		return nil
	}
}
func OptClientPoolSizePerRoute(size int) ClientOption {
	return func(options *ClientOptions) error {
		options.PoolSizePerRoute = size
		return nil
	}
}
// OptClientPoolSizePerRoute is the maximum number of active connections in the pool to a host.