I have a dashboard that displays data for a few databases. I set it up to be able to choose a db and display data for a single db. Now, in addition, I want to be able to display data for all of the databases at once.
To display a single db, I use a variable $database:
There is a Custom all value setting on variables. You have not said what your data source is, but if it supports wildcards then you can use the wildcard as a Custom all value.
So for example with Prometheus data source the wildcard is .* if I put that in Custom all value then when selecting your metric it would be: