promtool: Allow inexact matches in exp_alerts

Proposal

Use case. Why is this important?
Currently, when building a rule test, one needs the exp_alerts to be exact matches of the alert that would fire, with all labels and annotations. Since these can be fairly rich in real-world usage, I propose that matching can be "inexact". Rather than all labels and annotations being exact matches, only those actually present in the exp_labels and exp_annotations must match.
I frequently come across organisations where entire runbooks are written in annotations, and duplicating those into each rule adds a lot of friction to test adoption. Fixing a typo in the runbook breaking unit tests, for example.

This could be made opt-in with a matching_mode or such, if there is a frequent desire to match all the things.

Alternatively, or perhap as a complement, being able to check for presence but ignoring content. This could facilitate something like a test that validates that all alerting rules have a runbook, for example.