Proposal: Allow variable interpolation in generator values field

[Sinjo]

Context

While using the cluster generator, we recently ran into a situation where being able to template the contents of the values map based on the parameters of each cluster.

The specific situation was that we wanted to make it possible for devs to opt into deploying an environment-specific image from a branch, somewhat like this example from the docs, but where the environment and branch are per-developer.

For anyone not opted in, they hit a different selector and fall back to images built from the main branch.

Current workaround

Ideally, we would be able to construct the image URL as a value based on the cluster parameters (we're using the environment name for it). That would make it easy to supply a different, hardcoded value of main for any deployments that aren't opted into environment-specific images.

The workaround we're currently using is to have different generator templates on the two selectors, which leads to a fair amount of duplication due to the way the merge algorithm works.

Proposal

We would like to add the same parameter templating that's available in in the template itself to the map values in the values key of the selector. Running with the example from the cluster generator docs, it would look something like:

spec:
  generators:
  - clusters:
      selector:
        matchLabels:
          type: 'dev'
      # A key-value map for arbitrary parameters
      values: 
        revision: "dev-{{metadata.labels.environment}}" # dev clusters use per-environment branches
  - clusters:
      selector:
        matchLabels:
          type: 'staging'
      # A key-value map for arbitrary parameters
      values: 
        revision: HEAD # staging clusters use HEAD branch
  - clusters:
      selector:
        matchLabels:
          type: 'production'
      values: 
        # production uses a different revision value, for 'stable' branch
        revision: stable
  template:
    metadata:
      name: '{{name}}-guestbook'
    spec:
      project: "default"
      source:
        repoURL: https://github.com/argoproj/argocd-example-apps/
        # The cluster values field for each generator will be substituted here:
        targetRevision: '{{values.revision}}'
        path: guestbook
      destination:
        server: '{{server}}'
        namespace: guestbook        

---

If you're on board with the idea, I'm happy to go ahead and put a PR together that adds this functionality to the controller!

[Sinjo]

Hey, sorry for the bump. Just want to check if you're on board with this before I write code for it.

@jgwest can I take the addition of the enhancement label as a 👍🏻?