keep port custom_fields in place across device discovery

lib/App/Netdisco/Worker/Plugin/Discover/Properties.pm

@@ -371,6 +371,12 @@ register_worker({ phase => 'early', driver => 'snmp' }, sub {
   vars->{'hook_data'}->{'ports'} = [values %deviceports];
 
   schema('netdisco')->resultset('DevicePort')->txn_do_locked(sub {
+    # backup the custom_fields
+    my @fields = grep {exists $deviceports{$_->{port}}} $device->ports
+      ->search(undef, {columns => [qw/port custom_fields/]})->hri->all;
+    $deviceports{$_->{port}}->{custom_fields} = $_->{custom_fields}
+      for @fields;
+
     my $gone = $device->ports->delete({keep_nodes => 1});
     debug sprintf ' [%s] interfaces - removed %d interfaces',
       $device->ip, $gone;