feat: Add hasLabelsForEveryAggregation validator

[pw-mdb]

This adds a hasLabelsForEveryAggregation validator, which verifies that every aggregation in an alert expression has a set of labels. The new validator has a excludeRuleLabels option, which ignores labels in labels if they are specified in the alert rule's labels.

This is useful for partially ensuring that alert rules return labels that are required for AlertmanagerConfig matchers. For example, when using prometheus-operator, the operator adds an implicit namespace matcher to every AlertmanagerConfig. Alert rules that don't return the namespace label get silently blackhole'd. Because of this, aggregations are a huge source of blackhole'd alerts for us.

[FUSAKLA]

Hi! Thanks for the contribution 🎉

The validation idea makes sense, so I'd be happy to add it 👍

Could you please:

• fix the linter errors
• add tests same as other validators https://github.com/FUSAKLA/promruval/blob/master/pkg/validator/validator_test.go#L350
• add documentation for the new validator here https://github.com/FUSAKLA/promruval/blob/master/docs/validations.md
• (optional) add its usage to the e2e tests (so even loading configuration is tested) https://github.com/FUSAKLA/promruval/blob/master/examples/validation.yaml