update-debsources: nicer failure mode if metrics plugin is not enabled
update-debsources: nicer failure mode if metrics plugin is not enabled

total size will be set to 0 in that case

vivekanand1101/debsources,devoxel/debsources,devoxel/debsources,nonas/debian-qa,oorestisime/debsources,zacchiro/debsources,matthieucan/debsources,nonas/debian-qa,zacchiro/debsources,devoxel/debsources,clemux/debsources,nonas/debian-qa,vivekanand1101/debsources,Debian/debsources,zacchiro/debsources,sophiejjj/debsources,oorestisime/debsources,devoxel/debsources,matthieucan/debsources,devoxel/debsources,Debian/debsources,matthieucan/debsources,vivekanand1101/debsources,sophiejjj/debsources,vivekanand1101/debsources,clemux/debsources,Debian/debsources,oorestisime/debsources,Debian/debsources,nonas/debian-qa,clemux/debsources,clemux/debsources,zacchiro/debsources,sophiejjj/debsources,clemux/debsources,zacchiro/debsources,matthieucan/debsources,oorestisime/debsources,sophiejjj/debsources,oorestisime/debsources,vivekanand1101/debsources,Debian/debsources,sophiejjj/debsources,matthieucan/debsources