pool.go
func (pool *ReaderPool) Len() int {
	pool.mu.Lock()
	defer pool.mu.Unlock()
	return len(pool.givenOut)
}
func (pool *ReaderPool) Len() int {
	pool.mu.Lock()
	defer pool.mu.Unlock()
	return len(pool.givenOut)
}
// Len is the number of currently given out throttled readers.