This term unites the connotation of your typical metropolitan person (who for example talks inanely about mortgages, his "career", the latest iPhone/MacBook Pro/etc, how oppressed Africa is, how 'vibrant' high crime areas are or how the NHS has pretty much saved the world and "doesn't even cost that much"), with the blunt, directness of calling some one after the glans penis or in laymans terms - "bellend".
Nothing to do with Paris' underground railway network.
EXAMPLE 1: "I can't do this any more. I want to stick this shoreditch starbucks apricot frappacino straw in my eye. I'm just completely surrounded by metrobells".
"I used to be bullied at school because I believed in fair taxation. Do you know what they called me? A massive metrobell".