Related to #2409, add a statistic counter to track clients matching a HR that cannot be allocated because already in use

Per Support ticket #20651 and #2409 (closed), please could you add a new counter in the Kea stats:

Do you think its posible to add a new stats counter for ip-adresses conflicts? - I think it could be nice to have a counter for that, since I only noticed this problem then the HA-state flaps. 
But also have a easy method to catch any misconfigured dhcp-clients in the future.

This sounds like an excellent idea to me, especially because fixes for both #2409 (closed) and #2408 (closed) will have the effect of masking that 'something didn't quite work out as designed'.