expose BytesWritten via `LogExtraAttrs`

Currently, there is no way to access the bytes written that is tracked by httplog. The only approach is to have a separate middleware that wraps the response writer but this results in duplicating logic/processing that httplog already contains.

The use case around this is enriching logs based on the response size and/or performing usage based tracking.

Would solicit input on the preferred approach to implement, but happy to submit a PR if there is an appetite to add this.