PCT-635: Fix bool metrics

https://github.com/percona/mysql-log-parser/commit/eda1895c9615e968b9cd8d51147f388ed22c121c

http://play.golang.org/p/kcxppZ0GGo
```go
package main

import "fmt"

func main() {
	uint8 := uint8(253)
	fmt.Println("uint8: ", uint8)
	for i := 1; i <= 5; i++ {
		uint8++
		fmt.Println("uint8: ", uint8)
	}
}
```
```
uint8:  253
uint8:  254
uint8:  255
uint8:  0
uint8:  1
uint8:  2
```