why the body_bytes_sent is not equal to response content-length?

## Describe the bug

as title

e.g., body_bytes_sent=664, but the header content-length in response is actually 311.

## Pingora info

Please include the following information about your environment:

**Pingora version**: 0.8
**Rust version**: 1.94
**Operating system version**: macos 26

## Steps to reproduce

session.body_bytes_sent() in `logging` function
