Dashboards: Add template variables to selectable options

[fabrizio-grafana]

Add template variables to the options of Select fields for the Search tab for the Tempo datasource. Example:

Fixes #74210

[joey-grafana]

Overall feel like this a solid improvement 👍

Tag values would show No options found until the user had selected a tag key. I like that approach as the users knows that the values map to the key selected. Here we are pre-populating the values with the template variables. Perhaps it would improve UX to only show the template variables in the tag values when a tag key is selected? I'm not overly convinced of this approach but am not thinking of another this minute!

[joey-grafana]

Improvements look good 👍 You may be working on this, what do you think about my comment regarding No options found?

[fabrizio-grafana]

Yeah I wanted to have another thought on that today, because I also don't have a strong opinion on whether to keep the No options found entry until a scope is selected, or to already show the template variables.

[joey-grafana]

Yeah, I'd say it's not a necessary change to make right now.