Fix response size sum in metrics

[bullet4791]

Fix response size metric aggregation

The http_server_response_size_bytes_sum metric was incorrectly reporting zero because writtenBytes was only incremented on the first Write call, when !rrw.written. This ignored all subsequent writes, leading to inaccurate or zero size reporting, especially for responses written in multiple chunks.

Updated the Write hook to always increment writtenBytes by the actual number of written bytes (n), not len(b), and to track the first non-zero write separately. This ensures accurate response size measurement regardless of write pattern.

Changes:

• Fix incorrect writtenBytes accumulation
• Use n (actual written bytes) instead of len(b)
• Preserve written flag logic for WriteHeader tracking

[bullet4791]

@riandyrn can you check this PR?

[ilhamsyahids]

Hello @bullet4791

Thanks for fixing this, I was able to confirm the issue as well and the change is makes sense

Before merging this, could you also add a test case that covers this scenario? Specifically, it would be good to have a test where the response is written in multiple Write calls to ensure metric correctly accumulates the total bytes written which is should be FAIL before this patch.

[bullet4791]

Hello @ilhamsyahids
I've added the necessary tests. Can you check it?

[ilhamsyahids]

LGTM 👍