Draft dhcp scope stats

[englm]

What this PR does / why we need it

This PR is about including DHCP scope statistic metrics in windows_exporter:

• AddressesFree
• AddressesFreeOnPartnerServer
• AddressesFreeOnThisServer
• AddressesInUse
• AddressesInUseOnPartnerServer
• AddressesInUseOnThisServer
• PendingOffers
• ReservedAddress

Example output

# HELP windows_dhcp_scope_addresses_free Number of addresses free on given scope on the DHCP server (AddressesFree)
# TYPE windows_dhcp_scope_addresses_free gauge
windows_dhcp_scope_addresses_free{scope_id="192.168.102.0",superscope_name=""} 50
# HELP windows_dhcp_scope_addresses_free_partner_server Number of addresses free on partner server based on the ownership assignment of the free address pool. Applies to a failover scope. (AddressesFreeOnPartnerServer)
# TYPE windows_dhcp_scope_addresses_free_partner_server gauge
windows_dhcp_scope_addresses_free_partner_server{scope_id="192.168.102.0",superscope_name=""} 20
# HELP windows_dhcp_scope_addresses_free_this_server Number of addresses free on this server based on the ownership assignment of the free address pool. Applies to a failover scope. (AddressesFreeOnThisServer)
# TYPE windows_dhcp_scope_addresses_free_this_server gauge
windows_dhcp_scope_addresses_free_this_server{scope_id="192.168.102.0",superscope_name=""} 30
# HELP windows_dhcp_scope_addresses_in_use Number of addresses in use on given scope on the DHCP server (AddressesInUse)
# TYPE windows_dhcp_scope_addresses_in_use gauge
windows_dhcp_scope_addresses_in_use{scope_id="192.168.102.0",superscope_name=""} 5
# HELP windows_dhcp_scope_addresses_in_use_partner_server Number of addresses leased/renewed by partner server. Applies to a failover scope. (AddressesInUseOnPartnerServer)
# TYPE windows_dhcp_scope_addresses_in_use_partner_server gauge
windows_dhcp_scope_addresses_in_use_partner_server{scope_id="192.168.102.0",superscope_name=""} 2
# HELP windows_dhcp_scope_addresses_in_use_this_server Number of addresses leased/renewed by this server. Applies to a failover scope. (AddressesInUseOnThisServer)
# TYPE windows_dhcp_scope_addresses_in_use_this_server gauge
windows_dhcp_scope_addresses_in_use_this_server{scope_id="192.168.102.0",superscope_name=""} 3
# HELP windows_dhcp_scope_pending_offers Number of pending offers on given scope on the DHCP server (PendingOffers)
# TYPE windows_dhcp_scope_pending_offers gauge
windows_dhcp_scope_pending_offers{scope_id="192.168.102.0",superscope_name=""} 0
# HELP windows_dhcp_scope_reserved_address Number of reserved addresses on given scope on the DHCP server (ReservedAddress)
# TYPE windows_dhcp_scope_reserved_address gauge
windows_dhcp_scope_reserved_address{scope_id="192.168.102.0",superscope_name=""} 1

Which issue this PR fixes

• fixes DHCP include scope statistics #1836
• fixes DHCP Free DHCP IPs #880

Special notes for your reviewer

Unmarshal of the get method's result has to be completed.
OperationUnmarshalCallbacks.InstanceResult does not support ValueTypeINSTANCEA at the moment.