Safely handle range vectors from prometheus api result

[ta924]

Describe the feature

Currently Flagger assumes that a successful api call to prometheus (which contains a result), will be able to access v.Value[1], which will result in a panic if the user configured a metric that returns a range vector. I understand that instant vectors must be used but that does make Flagger vulnerable. I would think a better option would be to evaluate if values were returned from the result and send an error the same as NaN or no values found.

Source: https://github.com/fluxcd/flagger/blob/main/pkg/metrics/providers/prometheus.go#L150

What problem are you trying to solve?

Better error handling

Proposed solution

Add Values in the prometheusResponse struct so it will be unmarshaled for evaluation. While iterating the Result of the api response, if Values are present return an error message without triggering a panic. I don't see any drawbacks to this solution outside of an additional failure scenario.

Any alternatives you've considered? No

Is there another way to solve this problem that isn't as good a solution?

The other option would be to evaluate if v.Value[1] is nil, if so return an error instead of a panic. It think it all comes down to how elaborate you want the failure message to be back to the user.

[aryan9600]

yeah, this sounds good to me as well. are you interested in implementing the solution?

[ta924]

@aryan9600 yes I will get a PR together for review