Correct OLTP typo

[benbarclay]

This change replaces OLTP with the correct abbreviation OTLP.
It also provides a fallback for when the OLTP env vars are still set.

This should resolve #192

[dsocolobsky]

Cool although I think it's confusing to keep both at the same time, perhaps I would make the breaking change, delete the old ones, and just use the new env vars in prod. I wouldn't think it would be too complex to re-run the clients with the new env vars.

[benbarclay]

I've only kept both as an intermediate step while the change is rolled out. If we want to make it a hard breaking change, then that's pretty trivial to do so as well.

EDIT: I'm actually going to advocate for keeping this the way it is to decouple the code change from the config change. It lowers risk and requires less changes at once, at the cost of requiring a second PR to clean it up once the config is migrated. If we believe that we can safely do the whole thing in one go, then I'm happy to stand corrected, but some of our config changes have been pending quite a while to go out.