Kea DHCP servers' observations should be reset to some initial values upon restart or reconfiguration

While setting up the test environment of Kea with Prometheus we have discussed that various observations exposed by Kea should be set to initial values when the server is started. One use case is that you start the Kea server which doesn't receive any traffic before you set queries in Prometheus/Grafana. Currently, Kea performs lazy initialization of the statistics such as pkt4-received etc. Therefore, the Kea exporter would not know which statistics is supported by Kea, because this statistics would not be returned before the traffic is sent to the Kea server.

It is not practical to set default values in the Kea exporter, because it would be hard to keep it with sync with the statistics supported by Kea.