The story
Jocular extension of shitload (“a very large amount”), absurdly treating it as a unit of measurement (which could have a metric counterpart). Regarding the synonymous phrases a shitload of and a ton of, the coordination of ton versus metric ton is most likely the precedent by which shitload versus metric shitload was jocularly inspired; compare also metric fuckton.
Etymology from Wiktionary, CC BY-SA 4.0.