Fix errors in clustermetrics validation

[eliihen]

This PR fixes the following error:

When using urlFrom in the prometheus destination, it only uses the url field and throws due to url being Nil:

$ helm dep up; helm template observability .
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Error: template: observability/charts/k8s-monitoring/charts/k8s-monitoring/templates/validations.yaml:9:6: executing "observability/charts/k8s-monitoring/charts/k8s-monitoring/templates/validations.yaml" at <include (printf "features.%s.validate" $feature) $>: error calling include: template: observability/charts/k8s-monitoring/charts/k8s-monitoring/templates/features/_feature_cluster_metrics.tpl:71:43: executing "features.clusterMetrics.validate" at <$destinationUrl>: invalid value; expected string

Use --debug flag to render out invalid YAML

[CLAassistant]

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[petewall]

Thanks for catching this. There's a real bug here. I think we'll need to think a bit more about this.

In the URL case:

- type: prometheus
  url: http://prom.example.com/api/prom

the OpenCost validation tries to build a url for OpenCost to send promql queries to.

In the URL From case:

- type: prometheus
  urlFrom: sys.env("PROM_URL")

It can't turn that into a recommendation. Because of the OpenCost Helm chart, I can't use the same urlFrom semantic. What needs to happen is an "escape hatch" part to the validation that just says:

You wanna configure OpenCost? This is what you'll need:

clusterMetrics:
  opencost:
    opencost:
      prometheus:
        username: <basic auth username>
        password: <basic auth password>
        external:
          url: <Prometheus query URL>

[eliihen]

Absolutely! However, keep in mind when building the template the chart currently crashes, and this fix will ensure that doesn't happen. So while not perfect, it does show the fallback <$destinationName Query URL>, which will be an improvement over a nondescript error.

[petewall]

This change: #1012
should help with the opencost validation. If the destination url doesn't exist, or it can't figure it out, it'll just give a more generic message.

[petewall]

Can you retest with the latest rc? It should handle things better now.