high memory usage when using `LogExtraAttrs`

When using `LogExtraAttrs` to add additional attributes to the log, the `reqBody` is passed as a string parameter. In the case of large request bodies, for example, file uploads, this can lead to high memory usage.

Additionally, in situations where `reqBody` isn't accessed within `LogExtraAttrs`, there is unnecessary processing overhead introduced with [TeeReader](https://github.com/go-chi/httplog/blob/master/middleware.go#L46).

Possibly adding an alternate callback for adding attributes to the log that doesn't involve the request body, and/or a way to opt-in/out of having access to the `reqBody`?

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