Support dataframes in Alerting

[bergquist]

Currently alerting only supports time-series result (we dont want to change this requirement right now). To support the next generation back end plugins we need to support time-series data sent as data frames. Since data frames can contain any kind of data so we need to parse it and make sure to only pass series that contain time-series data to Alerting.

The alerting query is passed to the tsdb package at https://github.com/grafana/grafana/blob/master/pkg/services/alerting/conditions/query.go#L157
then all resulting series are added to the result at https://github.com/grafana/grafana/blob/master/pkg/services/alerting/conditions/query.go#L171
Here I think we should parse the data frames and add any time-series data to the result as well. This seems like the smallest possible change of introducing data frames in alerting.

Open questions

• What do we do with results that are not time-series data?

[bergquist]

Closed by #22812