Warn the user when they call the decorator without wrapping a function

[brettimus]

Right now, when func is None, we just return the sync_decorator.

We don't want to support this case, however, so it would be worthwhile to warn the user about calling the decorator without wrapping a function.

See: https://github.com/autometrics-dev/autometrics-py/pull/33#discussion_r1189501262

[brettimus]

this issue was filed due to a misunderstanding of how decorators with objectives (or additional args) would work

closing as it is no longer relevant