ALLOC_ENGINE_V4_ALLOC_FAIL_CLASSES via statistics API

In https://support.isc.org/Ticket/Display.html?id=18994 Support customer would like a new API function to obtain statistics on allocation failures.

These events are currently logged, and show as:

2021-08-18 09:21:38.256 WARN [kea-dhcp4.alloc-engine/480.140451601748032] ALLOC_ENGINE_V4_ALLOC_FAIL [hwtype=1 xx:xx:xx:xx:xx:xx], cid=[01:xx:xx:xx:xx:xx:xx], tid=0x4f2c435d: failed to allocate an IPv4 address after 3 attempt(s)

2021-08-18 09:21:38.256 WARN [kea-dhcp4.alloc-engine/480.140451601748032] ALLOC_ENGINE_V4_ALLOC_FAIL_CLASSES [hwtype=1 xx:xx:xx:xx:xx:xx], cid=[01:xx:xx:xx:xx:xx:xx], tid=0x4f2c435d: Failed to allocate an IPv4 address for client with classes: ALL, HA_server1, UNKNOWN