expfmt: Add UTF-8 syntax support in text_parse.go

[fedetorres93]

This PR adds support for the new UTF-8 syntax proposed here.

Main changes:

• Metric names may include UTF-8 characters and, if so, they will be quoted and inside brackets, without an operator (for example: {"my.metric", label="value"} 1)
• Label names may include UTF-8 characters and, if so, they will be quoted (for example: {"my.metric", "my.label"="value"} 1)

Addresses #554

[beorn7]

Thanks for doing this.

@ywwg mentioning you here in case you haven't seen it. You are probably more qualified to review this than me.

[beorn7]

All code paths in the if clause above end up returning, so we don't need the else here.

[beorn7]

I know that protobuf uses the singular even for repeated messages. But maybe it would still make more sense and be more readable to use the plural here? currentLabels collides with the map of the same name below, but maybe call this currentLabelPairs? Also, is there a good comment to add here to explain what this variable is for? (cf. the comment for the currentLabels variable below).

[beorn7]

Nit: This should end with a period to be consistent with the naming above.

More substantially: Let's be more explicit here about what's significant. It's not special that this is a counter, and "UTF-8" per se doesn't say anything (the old metric names were all valid UTF-8, too). How about "Quoted metric name and quoted label name with dots."?

[beorn7]

Suggestion: "Metric family with dots in name."

[beorn7]

As above, end with period.

[beorn7]

As above, "gauge" is not important. Say something about the special character rather than just stating "UTF-8".

[beorn7]

We need more test cases. Ideas:

• Various escaped special characters in a label name and a metric name (like \", but also including multi-line with \n).
• The name is not the first element: {foo="bar", "late.name"}.

It would be good to also add a few more cases to testTextParseError, e.g. multiple metric names like {"one.name", "another.name"}.

[fedetorres93]

@beorn7 Thank you for your review. All feedback has been addressed.

[beorn7]

LGTM. @ywwg do you want to give it a final look?

[ywwg]

have you run code coverage on this to make sure the major branches are covered? Unless something is missing, lgtm

[fedetorres93]

have you run code coverage on this to make sure the major branches are covered? Unless something is missing, lgtm

Most branches were covered, but I added a couple more tests to cover additional error cases.

[ywwg]

thank you for the last tests! lgtm

[beorn7]

Thank you both. I'll squash this because we have quite a few mini commits in here.