packet statistics should use a counter instead of gauge

[M0NsTeRRR]

Packet statistics currently use a gauge, but they should use a counter instead. Kea only increments values for packet statistics since there's a command to reset them to 0.

Kea reset command
Prometheus documentation

[Qwiko]

Is there other metrics that also could be represented by a counter? Maybe there are some other metrics that could benefit using a counter instead.

@M0NsTeRRR do you want to work on this and provide some code?

[M0NsTeRRR]

I let you review my linked PR @Qwiko :)