I don't want to be cynical but... did this really need to be a thing? What sort of text editor doesn't include this out of the box?
Perhaps he wanted to make it for fun, hard to believe, I know.
Hmm, micro service? why not?
I suppose I can ask "why?" just as easy as you can ask "why not?".
Photoshop doesn't have it, so I sometimes use a web based character counter to get a basic sense of line length. This one seems pretty neat for that.
I did something similar with a bit more functionality (mostly exporting to Google Docs).
What inspired you to make yours?