Implement WriteThroughput metric for sync clients

[zoewangg]

Motivation and Context

Add WRITE_THROUGHPUT metric to measure request body upload speed (bytes/sec) for sync clients.

This metric helps customers understand upload performance and diagnose throughput issues. The implementation uses tracking last read time which provides more accurate throughput measurement than TTFB-based approaches, especially on fast networks where server processing time can significantly skew results.

Note: This PR targets the feature branch. Async client support will be added in a follow-up PR to the same feature branch,
and the feature branch will be merged to master after both sync and async implementations are complete.

Modifications

• CoreMetric.java - Added WRITE_THROUGHPUT metric with comprehensive Javadoc explaining formula, exclusions, and buffering caveat
• SdkInternalExecutionAttribute.java - Added REQUEST_BYTES_WRITTEN, REQUEST_BODY_FIRST_BYTE_WRITTEN_NANO_TIME,
  REQUEST_BODY_LAST_BYTE_WRITTEN_NANO_TIME
• BytesWrittenTrackingInputStream.java - New class that tracks bytes written and timing (follows BytesReadTrackingInputStream pattern)
• ApiCallAttemptMetricCollectionStage.java - Initializes tracking counters before each attempt
• MakeHttpRequestStage.java - Combined trackBytesWritten and enforceContentLengthIfPresent into single wrapRequestContentStream method to wrap
  stream once
• HandleResponseStage.java - Reports WRITE_THROUGHPUT metric in new reportWriteThroughput method
• MetricUtils.java - Added helper methods for accessing write metric attributes

Formula: WRITE_THROUGHPUT = RequestBytesWritten / (LastByteWrittenTime - FirstByteWrittenTime)

Testing

• Unit tests: BytesWrittenTrackingInputStreamTest - 11 tests covering all read methods, edge cases, and timing behavior
• Functional tests: SyncWriteThroughputMetricTest - WireMock-based JUnit 5 tests verifying metric is reported for requests with body and not
  reported for requests without body
• Integration test: PutObjectIntegrationTest.putObject_withRequestBody_reportsWriteThroughputMetric - Validates real S3 upload reports the metric

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the
  instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated [LaunchChangelog](https://github.com/aws/aws-sdk-java-v2/blob/master/docs/
  LaunchChangelog.md)

License

• I confirm that this pull request can be released under the Apache 2 license

[sonarqubecloud]

Quality Gate passed

Issues
22 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud