add s3nd_upload_attempts histogram metric

[jhoblitt]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a new histogram metric to track the number of upload attempts per file transfer operation. The metric provides insight into upload reliability and potential retry patterns.

• Introduces s3nd_upload_attempts histogram metric with native histogram configuration
• Records the metric value from task.UploadAttempts during HTTP request processing
• Updates test expectations to include the new metric in validation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
upload/handler.go	Defines the new histogram metric and records its value during request handling
test/metrics_test.go	Adds the new metric name to test validation list

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