Ability to retrieve evaluation value from Threshold expression

[gelicia]

Discussed in https://github.com/grafana/grafana/discussions/73288

^{Originally posted by **gtirloni** August 15, 2023} Through the `$values` variable, we are able to access the result of all the queries/expressions in an alert. However, we are not able to retrieve the actual value defined in a Threshold expression. Only the result (one or zero) is visible. It would be useful to have the actual evaluation value from a Threshold expression so we can use it in a template (e.g. to dynamically define a summary that goes to the notification).

[grobinson-grafana]

Discussed with the team, we are considering introducing variables into the rule definition that could then be used in both the expressions (such as Threshold) and the annotation templates. We think this would address this use case, but we haven't committed to building this for the time being.