feat: topk check

[wbollock]

This adds a new check to warn against the use of topk or bottomk in
recording rules. This is an anti-pattern as these operators lead to high
churn as the time series the recording rule generates will change
frequently as the conditions for topk/bottomk adjust.

It is enabled by default with a warning severity. It will only fire for
recording rules, not alerting rules.

Resolves #820

[prymitive]

There is nothing wrong with using topk() in rules as long as the rule doesn't just return topk() results verbatim. Consider this query: min(topk(10, ...)) > 5000 it returns no labels and so will produce stable results.
Just looking for any topk use in a query is not enough to produce useful results here.

On top of that this code uses regex matchers to find uses of topk string, instead of looking for actual usage of topk() function using the AST. That's fragile and prone to false positives so sorry, but I won't be adding it.

[wbollock]

no worries, that makes sense. thanks for looking