add http status labels to s3nd_upload_bytes_total metric

[jhoblitt]

And always observe the value, even upon failure.

[Copilot]

Pull Request Overview

This PR modifies the s3nd_upload_bytes_total metric to include HTTP status labels and ensures it's observed for all requests, not just successful ones.

• Changes the metric from a simple Counter to a CounterVec with "code" and "reason" labels
• Moves the bytes counting logic to always execute regardless of upload outcome
• Groups metric observations with comments for better code organization

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