Add down sample to example query

Currently the example query just truncates the results to `max_data_points`.
```
# this seems to be optional, play with it and see what happens.
LIMIT max_data_points;
```

Instead the result should be down sampled to `max_data_points`.

See also https://grafana.com/docs/grafana/latest/panels-visualizations/query-transform-data/#query-options

Locate the example query containing the shown LIMIT max_data_points snippet and read how its results are currently produced. Consult the linked Grafana query-options documentation for the expected downsampling behavior; done means the example query downsamples results to max_data_points instead of merely truncating them.

Written by the indexing model from the issue text.