Fix awss3reader prefix trim

[anhtaw]

Description

Fixing a bug in the awss3receiver key generation logic where object keys could contain
an unexpected leading slash (/year=...) when s3_prefix was empty and s3_partition_format
was not explicitly set.
This change makes the behavior consistent between default and custom s3_partition_format
by ensuring keys do not start with a slash unless explicitly configured.

Additional logic was added to handle cases where prefix equals / or //, preserving the
user-defined format without stripping or duplicating slashes.

---

Link to tracking issue

Fixes #43522

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Testing

• Added new unit tests in s3reader_test.go covering:
  • Default s3_partition_format behavior with empty prefix.
  • Explicit s3_partition_format override.
  • Custom prefix values / and //.
• Verified generated S3 keys match expected format:
  • year=2025/month=10/day=15/... (no leading slash)
  • /year=2025/... when prefix = /.
• All existing tests pass (make test).

---

Documentation

No external documentation changes required.
Internal behavior change documented in code comments for getObjectPrefixForTime().

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: anhtaw / name: broname (bdbca67)

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[codeboten]

Thanks for the contribution @anhtaw! please ensure a changelog entry is added https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#adding-a-changelog-entry

[anhtaw]

Thanks for the contribution @anhtaw! please ensure a changelog entry is added https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#adding-a-changelog-entry

Hi @codeboten Changelog entry has been added. Thanks for the reminder!

[adcharre]

Looks OK to me and seems to follow similar behaviour as to the S3 Exporter.

[github-actions]

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Important reminders:

• Please review our Contributing Guidelines.
• Don't forget to sign the Contributor License Agreement (CLA) if you haven't already.

A maintainer will review your pull request soon. Thank you for helping make OpenTelemetry better!