Different values for interval based on datasource

**What would you like to be added**: I'd like to have different values for interval depending on the datasource. **Why is this needed**: In a `grafana -> cortex (cache) -> thanos query (reverse proxy) -> thanos store / prometheus (data sources)` context we can query raw original data up to a certain period and then we can have 5m and/or 1h downsampled data. To be able to chose between what kind of resolution I want I have created three data sources: - thanos (for raw data with a scrape interval of `60s` which is prometheus default scrape interval) - thanos-5m (for 5m downsampled data, with a scrape interval of `5m`) - thanos-1h (for 1h downsampled data, with a scrape interval of `1h`) My dashboards have a `$delta` variable which is an interval type which allows to chose the range vector length in most of the dashboard queries involving deltas, i.e.: ``` rate(my_vector{labels=~"$label"}[$delta]) ``` $delta values are `2m,5m,10m,30m,1h,6h,12h,1d`. With raw data this works well but when using downsampled data sources it does not work any more if the **$delta is lesser than or equal to the downsampled resolution**. In that case we get graphs with holes, e.g.: <img width="1665" alt="k8s-pods-2-thanos_-_Grafana" src="https://user-images.githubusercontent.com/153052/73056721-0b0f8280-3e90-11ea-85b9-c8f300fb643a.png"> <img width="1668" alt="k8s-pods-2-thanos_-_Grafana2" src="https://user-images.githubusercontent.com/153052/73056746-182c7180-3e90-11ea-9738-19e02c2f0c7b.png"> I myself does not really mind because I know that I have to use higher deltas with downsampled sources (and why I have to do it). The problem comes from my coworkers whom all struggles with that and always complain about buggy graphs. It would be nice to have different interval values depending on the datasource so that I can make sure people will not use values for $delta that could not work. Regards.