Explore metrics: Scenes variables spacing fixes

[bohandley]

When using OTel, the spacing between the Scenes variables is odd

From @catherineymgui

Right now there's 16px padding between label values selectors, but then only 8px padding between the add attribute buttons, which makes it seem like the buttons are more closely related than the labels. Could we switch up the padding (16px padding between buttons, 8px padding between label value selectors)?

We will need to update Scenes variables to allow for CSS updates.