pool.go
func (pool *WriterPool) Len() int {
	pool.mu.Lock()
	l := len(pool.givenOut)
	pool.mu.Unlock()
	return l
}
func (pool *WriterPool) Len() int {
	pool.mu.Lock()
	l := len(pool.givenOut)
	pool.mu.Unlock()
	return l
}
// Len is the number of currently given out throttled writers.