I might call it a flanged bushing, except that searching for that term comes up empty. In the past, I posted an answer to a question that is unrelated to this one, but for the result:
It's called a steel protector sleeve and common sizes from my search are 1/4" inch and smaller inside diameter. M6 is close to 1/4" but is not M6. The next common size downward is 3/16" which is 4.7 mm, perhaps closer to M5. This may be a result of my searches based in the USA, or that metric versions do not exist, which is somewhat absurd to consider.
I popped off a quick search to McMaster-Carr with results appearing for flanged sleeve bearings, which are available in M5 inside diameter but are lubricated for taking rotational and axial loading.