Ensure chart panels in Metrics dashboard are focusable with keyboard
Problem to solve
Currently, the chart panels in the Metrics dashboard can't be focused on by tabbing on a keyboard. That makes the whole dashboard absolutely inaccessible, as well as blocks our implementation of keyboard shortcuts that work on focused chart panels.
We should make the chart panels accessible by tabbing using keyboard. That will enable us to implement keyboard shortcuts and take us one step closer to making GitLab Metrics accessible to users who can't use a mouse.
This issue may be a good opportunity to look how tabbing currently works (or doesn't) to access chart menus, dashboard controls etc.