Escape newlines in LogfmtRenderer

[madjar]

Summary

Currently, the LogfmtRenderer doesn't escape newlines, so any newline in a field (most commonly exceptions, but also other fields) ends up breaking the log over multiple lines, which messes up with log processing downstream.

This PR makes it so that newlines get escaped. I've checked that the behaviour is identical to the implementations in https://github.com/jteppinette/python-logfmter/ and https://github.com/sirupsen/logrus (which are the other logfmt implementations I happen to be using).

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[hynek]

Ah great, a logfmt expert! Any opinions about #511 / #513? 🥺

[hynek]

Thanks!

[madjar]

I would not call myself an expert, but I'm happy to take a look at compare the behaviour of the various tools in my stack :)