database filter in slow query page doesn't work very well.

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

What did you do?

1. create a new tidb cluster.
2. set @@tidb_slow_log_threshold=0;
3. Use dashboard to query slow log.
4. Use database filter to filter slow query:
   

What did you expect to see?

some slow query like this:

What did you see instead?

nothing

What version of TiDB Dashboard are you using (./tidb-dashboard --version)?

[breezewish]

The filter is "execution database" as shown in the UI. We can discuss whether execution database should be changed to something like related database.