feat: Use GitLab design system spacing in gl-form-group

The gl-form-group inherits the spacing between elements from form-group from bootstrap. This MR aims to

• Adapting the gl-form-group to use GitLab pixel spacing.
  • This results in slight visual changes in the spacing.
• Reduce CSS from metrics screens in Gitlab CE.
  • Comes from changes related metrics screen url: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31239/diffs?diff_id=50464731&start_sha=7186a5274758a71ceee31e60efd529d27cd4d47d#7fd39c2ef43416a3d24ef5599813bdf322b5341e_7_8

2 examples of usage in context (EE -> Prometheus Integration -> New metric):

before	after