Added `theme-service-init` metric during boot
Added `theme-service-init` metric during boot

refs https://github.com/TryGhost/Toolbox/issues/254

- it's useful for us to know how long it takes the theme service to
  init, especially as it is dealing with a lot of user-provided files
  outside of the Ghost codebase

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost