Add "selected" field to custom template variable when multi=false

[DifferentialOrange]

Adds "selected" field to custom template variable when multi=false. Select first value as current if current is not specified.

Change was introduced in #222 merge request, reworked due to #232 merge request contribution.

In current version of grafonnet-lib "selected" field added to custom template variable only when multi is true. But Grafana behavior is different.

If we create simple custom variable

Grafana's dashboard code will contain the following object:

{
    "allValue": null,
    "current": {
      "selected": false,
      "text": "1",
      "value": "1"
    },
    "hide": 0,
    "includeAll": false,
    "label": null,
    "multi": false,
    "name": "custom",
    "options": [
         {
            "selected": true,
            "text": "1",
            "value": "1"
         },
         {
            "selected": false,
            "text": "10",
            "value": "10"
         }
     ],
    "query": "1, 10",
    "skipUrlSync": false,
    "type": "custom"
}

i.e. Grafana adds "selected" field even if "multi" is false and, moreover, selects first value as current if user don't set current value. The behavior is the same if "Include All option" is enabled.

[bcdady]

I see that this was pointed to in favor of #222, however this has not yet been merged.
Is there something else that needs to happen to get this merged?
It will also be very helpful to see an example implementation.

[bcdady]

@DifferentialOrange, can you please elaborate on how/why this is favorable over #222?
How does this MR/PR offer advantages for defining a template variable with type='constant' in grafonnet?

[DifferentialOrange]

@DifferentialOrange, can you please elaborate on how/why this is favorable over #222?

As far as I remember (sorry, two years have passed) #222 was separated into two independent PRs: #257 and #258, considering up to date master. #257 and #258 should introduce the same functionality as #222, it's more like a formalization.

How does this MR/PR offer advantages for defining a template variable with type='constant' in grafonnet?

It doesn't. This PR is about custom variable. For constant variable, refer to #257

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