This trigger expression works as follows:
1. It can be triggered if the operations status is Null (1) or Unknown (0).
2. `{$DELL.HTTP.IFCONTROL:"{#IFNAME}"}=1` - a user can redefine the context macro to "0", marking this interface as not important. No new trigger will be fired if this interface is Null (1) or Unknown (0).
3. `{TEMPLATE_NAME:METRIC.diff()}=1` - the trigger fires only if the operational status was up to (1) sometime before (so, does not fire for the "eternal off" interfaces.)

WARNING: if closed manually - it will not fire again on the next poll because of `.diff`.