Create Prometheus metrics in Application controller to track sources updates

[panzouh]

Summary

In general, Helm charts are resilient to updates, value files and templates differ little from one version to another. However, this is not always true as I have already tested the tracking strategies (https://argo-cd.readthedocs.io/en/stable/user-guide/tracking_strategies/#helm). Sometimes the value files require refactoring. That's why I think it would be interesting to have a Prometheus metric that tracks version changes. In the past I could also test a similar tool (https://github.com/sstarcher/helm-exporter/), which was quite handy, unfortunately the Helm plugin being a helm template command instead of helm upgrade --install, it is impossible for me to use it (as mentioned in issue #8591).

Proposal

It would be enough either to change the Helm generation command (issue #8591), or create argocd_app_outdated metric :

argocd_app_outdated{app="sample-chart",currentVersion="1.2.1",latestVersion="1.2.3",namespace="sample",release="sample-chart"} 1