Avoid extra quotes where not strictly necessary.

It's not necessary to enclose in quotes every single string value
in log2met format; when using basic words, it's possible to not
quote it (as heroku does for its own logging). This keeps the
logs easier on the human eye.
Hm, is it possible to avoid a regex altogether here? I can't really come up with anything that doesn't require something that's even harder to read.