Add `promql/gauge_staleness` check

Frequently we make nits to users about using query functions when alerting on gauges:

https://www.robustperception.io/alerting-on-gauges-in-prometheus-2-0/

I believe this would be a nice addition to Pint and Prometheus rule linting. Based on the counter check it seems like we could also find if a metric is a gauge and not covered by a query function: https://github.com/cloudflare/pint/blob/63fb30627bd644f2c3ed86e073bcae98badf6a56/internal/checks/promql_counter.go#L51

I'm happy to contribute this back to Pint but want to discuss with the maintainer if this would be acceptable

cc: @prymitive