histogram.go
func (db *DB) PrintHistogram(keyPrefix []byte) {
	if db == nil {
		fmt.Println("\nCannot build histogram: DB is nil.")
		return
	}
	histogram := db.buildHistogram(keyPrefix)
	fmt.Printf("Histogram of key sizes (in bytes)\n")
	histogram.keySizeHistogram.printHistogram()
	fmt.Printf("Histogram of value sizes (in bytes)\n")
	histogram.valueSizeHistogram.printHistogram()
}
func (db *DB) PrintHistogram(keyPrefix []byte) {
	if db == nil {
		fmt.Println("\nCannot build histogram: DB is nil.")
		return
	}
	histogram := db.buildHistogram(keyPrefix)
	fmt.Printf("Histogram of key sizes (in bytes)\n")
	histogram.keySizeHistogram.printHistogram()
	fmt.Printf("Histogram of value sizes (in bytes)\n")
	histogram.valueSizeHistogram.printHistogram()
}
// PrintHistogram builds and displays the key-value size histogram.
// When keyPrefix is set, only the keys that have prefix "keyPrefix" are
// considered for creating the histogram