Well you have me- from proto-european which means to measure.

Then you have metrical (metricus/metrikos from Latin/Greek) that means to measure rhythm in poetry.

Mētrum/Metron again from Latin/Greek meaning "measure, length, size, limit, proportion"

Then "metre" which is originally a unit of length. Then you have a "metre stick" which is a stick used to measure a metre. You can blame the French for basically calling it a "measurement stick" but it refers to a very specific measurement.

Then you have the -or suffix in Latin which means "to have to do with" or "to pertain to". Then that turns in to -re and -er in Old English.

And like everything else - Brittan used both for centuries before deciding one was "right" and everyone else is at fault for the other way (just like how "Soccer" is a British term). Famously Shakespeare used both -re and -er.

Lastly, the US uses the metric system for its professions. It's layman's terms that don't use metric.