core: split $MONITOR_METADATA and return it only if a single unit triggers OnFailure...

core: split $MONITOR_METADATA and return it only if a single unit triggers OnFailure/OnSuccess

Remove the list logic, and simply skip passing metadata if more than one
unit triggered an OnFailure/OnSuccess handler.
Instead of a single env var to loop over, provide each separate item
as its own variable.

Fixes https://github.com/systemd/systemd/issues/22370