Exclude vector queries from being counted in metric for rules with zero fetched series

[zenador]

What this PR does

We have a counter metric that tracks rules with zero fetched series. This PR excludes queries that are just vector(number) from being counted in there, as those are not expected to fetch any series and are not an indication of late data that may affect the rules.

Which issue(s) this PR fixes or relates to

Follow up to #5925

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[dimitarvdimitrov]

i'd imagine there are other queries that fetch 0 series. Any constant expression or any of the promql functions that don't take any args (week() day_of_week() time()). Do we want to account for them too?

One way to do that is to parse the promql query and find if it has any vector selector. This an example of doing this in the query-frontend

mimir/pkg/frontend/querymiddleware/stats.go

Lines 49 to 63 in 3543b63

	if expr, err := parser.ParseExpr(req.GetQuery()); err == nil {
	for _, selectors := range parser.ExtractSelectors(expr) {
	for _, matcher := range selectors {
	if matcher.Type != labels.MatchRegexp && matcher.Type != labels.MatchNotRegexp {
	continue
	}
	s.regexpMatcherCount.Inc()
	if matcher.IsRegexOptimized() {
	s.regexpMatcherOptimizedCount.Inc()
	}
	}
	}
	}

[zenador]

Thanks, good suggestion!

[leizor]

The CHANGELOG has just been cut to prepare for the next Mimir release. Please rebase main and eventually move the CHANGELOG entry added / updated in this PR to the top of the CHANGELOG document. Thanks!

[dimitarvdimitrov]

LGTM, thank you!