optimize to posting group label value matching

[yeya24]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

The optimization comes from prometheus/prometheus#13958 and https://github.com/prometheus/prometheus/blob/main/tsdb/querier.go#L358.

With latest Prometheus regex matcher optimization, now empty value "" is optimized into emptyStringMatcher which only checks the value length https://github.com/prometheus/prometheus/blob/main/model/labels/regexp.go#L325.

// emptyStringMatcher matches an empty string.
type emptyStringMatcher struct{}

func (m emptyStringMatcher) Matches(s string) bool {
	return len(s) == 0
}

Since label values should be always non empty, we can optimize some cases to not run matching at all.

Verification

[GiedriusS]

Why do we need to check for len(values) == 1?

[yeya24]

I think the difference we have with prometheus/prometheus#13958 is that we cache results of label values. So we cannot reuse vals only if it is the last value we have. I have updated the condition.

[yeya24]

I am actually a little bit unsure now if it reuses the values array causes any issue or not.

Updated: OK I think it is not safe to reuse the values other than reusing it once at the last value.