Loggable builds log messages even if they aren't emitted, reducing performance.

Loggable currently builds messages eagerly, even if the message won't be emitted because of the logger's level setting. This is particularly bad because `log_inspect` uses `respond_to?` on every operation to be logged, which walks the Ruby module tree, which is slow and bad.

https://github.com/cheald/mongo-ruby-driver/commit/e187ab023d01ec256a3142053e23a12360af8f92

Using the same benchmark as my previous code, this improved my benchmark runtime by a full 10% (12.1 sec -> 10.9 sec).