Hide nonexistent combinations when filtering metrics on dashboard

When I filter metrics on the dashboard, I'd like the UI to indicate which other filters I can add without excluding all measurements.

For example, let's say a service outputs metrics with these tag combinations:

- server=koraktor, sensor=cpu
- server=koraktor, sensor=memory
- server=grail, sensor=cpu
- server=grail, sensor=power_supply

Then if I set a filter on "sensor=memory" and click the "server" filter button, I'd like the dashboard to show that only "server=koraktor" has measurements available.

Something like in `xfontsel`.