enh: logginglogmetric: clarify unset for direct controller

[acpana]

We need to consider the unset intent for logginglogmetric as a direct controller.

That way a field that isn't specified is unmanaged (instead of forcing the value to empty).  It's a decision though, and we probably should reach an agreement.  I think that for the current Terraform controllers, we consider nil fields to be unset.

I do think it also makes it harder to clear a field though.

Originally posted by @justinsb in #1499 (comment)