add tracking of request body reads

Currently, [RequestBytes](https://github.com/go-chi/httplog/blob/master/middleware.go#L132) is logged, however it's based solely on `ContentLength`. This leads to potential inaccuracies if `ContentLength` header isn't present. Additionally, it doesn't track actual bytes read.

Would like to see logging attributes include the actual bytes read, possibly following an approach similar to [Caddy](https://github.com/caddyserver/caddy/blob/3b4d966fba069e9895980dbbf05f84bf818968ba/modules/caddyhttp/server.go#L343).

Happy to submit a PR if there is an appetite to add this.