Document difference between tab and newline in tests

[grobinson-grafana]

I found a possible bug (unexpected behaviour?) in the current parser for label matchers where \n is interpolated and \t is not. I've chosen to document this with a test while I figure out if this is intentional or not.

Related: #3353

[grobinson-grafana]

At least, from the PromQL documentation, I would expect \t and \n to behave the same:

"this is a string"
'these are unescaped: \n \\ \t'
`these are not unescaped: \n ' " \t`

[gotjosh]

Tagging @beorn7 and @simonpasquier to see if they have any reckons. In the meantime, I'll merge this as tests that document existing behaviour are always a great thing to have.

[beorn7]

This is probably inspired by the behavior in the exposition format, in particular: “… the backslash (), double-quote ("), and line feed (\n) characters have to be escaped as \, ", and \n, respectively” (i.e. this is not using the usual Go string escaping, but a very specific limited one).

This is certainly a mixup as the matchers are supposed to be inspired by PromQL, not by the exposition format. So your expectation is spot-on, the matchers should follow escaping rules of PromQL wherever reasonably possible.