Feature Request: Support selecting columns from resulting data in query

I'd like to use the Grafana expression language to [write alerting rules based on the output of the Github Datasource. Right now I can't because](https://grafana.com/docs/grafana/latest/panels-visualizations/query-transform-data/expression-queries/):
> Currently, the only non-time series format (number) is supported when you’re using data frames and you have a table response that returns a data frame with no time, string columns, and one number column

Queries for Issues and Pull Requests return at least one numeric field, and several other fields that aren't appropriate for labels. I'd like to be able to get data equivalent to this SQL call:
```
SELECT pull_request.open_time, pull_request.author FROM pull_request WHERE pull_request.open is TRUE;
```

Perhaps this could be done in the plugin by dropping fields from the data frames until only the requested fields remain?