Why not export `Objective`s from the top-level `autometrics` module?

[brettimus]

Right now, to import and use Objectives, I need to write

from autometrics import autometrics, init
from autometrics.objectives import Objective, ObjectiveLatency, ObjectivePercentile

Could we just change this so you can import everything from the top-level module?

E.g.,

from autometrics import autometrics, init, Objective, ObjectiveLatency, ObjectivePercentile