grafana: set `isDefault` to true for single prometheus data source

[jgehrcke]

Hello! Thank you for the great work on this project.

I'd love for the Prometheus in kube-prometheus to be configured as an isDefault: true data source in Grafana.

That parameter is documented with

    # <bool> Toggles whether the data source is pre-selected
    # for new panels. You can set only one default
    # data source per organization.
    isDefault:

(https://grafana.com/docs/grafana/latest/administration/provisioning/).

When trying to (re)use the same dashboard JSON definition across various Grafana deployments it makes sense to rely on a sane 'default' data source. In the dashboard JSON definition it can then be pre-selected with

        "current": {
          "selected": true,
          "text": "default",
          "value": "default"
        },

Currently, grafana-dashboardDatasources.yaml is generated by kube-prometheus with these details:

stringData:
  datasources.yaml: |-
    {
        "apiVersion": 1,
        "datasources": [
            {
                "access": "proxy",
                "editable": false,
                "name": "prometheus",
                "orgId": 1,
                "type": "prometheus",
                "url": "http://prometheus-k8s.monitoring.svc:9090",
                "version": 1
            }
        ]
    }
type: Opaque

That is, we do not define the isDefault parameter on the Grafana datasource.

I am looking at https://github.com/prometheus-operator/kube-prometheus/issues/1316#issuecomment-897460827 and there probably is a way to set the isDefault: true via my custom jsonnet stack configuration. (how would I do that?)

But in the longer run, I wonder: why not set isDefault: true by default here? When there's just one data source defined this cannot hurt.

Here's an example showing the "default" toggle in the UI: