Body length tags have a default value of "0", should they be "-1" now?

In our logging tags that provides a value for the body length, we default to 0 in the absence of a value. This is somewhat bad now, because we actually supports caching objects with a zero length body. Hence, it's not distinguishable between a zero length body entry, or one where we couldn't retrieve a body length (for whatever reason).

I'm wondering if we should change all the defaults for all "byte counts" in logging from "0" to "-1"? This would be an incompatible change, so marking this for v5.0.0.