Metric tonnes: People get confused if you just say tonnes because there are some traditional ton units that are of similar scale but different. So, "metric tonnes" avoids this confusion even when it isn't strictly necessary in written English.
If you're wondering why you'd measure hydrogen in tonnes at all, well, it's a measure of mass, you could tell people how many hydrogen atoms it is but that's some mind-bogglingly huge number so it just feels meaningless, a tonne of something you can imagine, even if doing so for hydrogen is a stretch.
A metric ton(ne), by the way, is the same quantity as the unambiguous and compositional, but rarely used, "megagram". (Oh, but kilotons and megatons are usually units of energy as shorthand for burning TNT.)
If you're wondering why you'd measure hydrogen in tonnes at all, well, it's a measure of mass, you could tell people how many hydrogen atoms it is but that's some mind-bogglingly huge number so it just feels meaningless, a tonne of something you can imagine, even if doing so for hydrogen is a stretch.
Or maybe I missed what the question is.