feat(GlSingleStat): allow unit prop to accept string or number

mentioned in commit db799836

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2263 (merged)

mentioned in merge request status-page!237 (merged)

mentioned in merge request gitlab-docs!1548 (merged)

mentioned in merge request gitlab!55107 (merged)

mentioned in commit 11d1ddeb