Epic: Support UTF-8 characters in Alertmanager label matchers

[grobinson-grafana]

What

The Prometheus maintainers have agreed to support UTF-8 label matchers in Alertmanager. This issue tracks the work being done to deliver this change in Alertmanager.

Background

To support UTF-8 label matchers in Alertmanager we have restricted the grammar for label matchers and written a new LL(1) parser to parse this grammar. Our motivation for doing this, instead of extending the existing regular expression parser, is that we found a number of ambiguities in the existing grammar and parser that would have been made worse if we added support for UTF-8 label matchers. You can find a presentation that explains these ambiguities here.

Examples of ambiguities

{foo=}} equivalent to {foo="}"}, but I think this should be an error
{{foo=} is an error because of two {{, unlike the above
{foo=~} could be either {foo=~""} or {foo="~"}, it's interpreted in current versions as {foo=~""}
{foo=,} is equivalent to {foo=""}, but I think should be an error as a comma with no value has a high likelihood of being human error
{foo=,,} is an error, unlike {foo=,} or {foo=}}
{foo= } is equivalent to {foo=""}
{foo= }b is equivalent to {foo="}b"}
{foo= b} and {foo=b } are equivalent to {foo="b"}, but {foo=b b} is equivalent to {foo="b b"}

Example	Can parse?
{foo=bar😊}	✅
{foo="bar😊"}	✅
{foo😊=bar}	❌
{"foo😊"=bar}	✅

Here bar😊 is allowed on the right hand side of the expression without quotes but not the left.

Tasks

• Support UTF-8 label matchers: Add new parser #3453
• Support UTF-8 label matchers: Add test for PromQL braces when parsing lists of matchers #3507 and Support UTF-8 label matchers: Add more acceptance tests for braces when using amtool #3523
• Support UTF-8 label matchers: Add compat package with feature flag and use in amtool #3483
• Support UTF-8 label matchers: Use compat package in Alertmanager server #3567