[Resources.AWS] Limit response body size read

[martincostello]

Changes

• Limit the length of the HTTP response body that is read from the AWS EC2/ECS/EKS instance metadata service.
• Simplify stream reading.
• Remove redundant sync-over-async calls.

Builds on top of #4117.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.86%. Comparing base (9d8a364) to head (9af5375).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry.Resources.AWS/AWSEC2Detector.cs	60.00%	2 Missing ⚠️
src/OpenTelemetry.Resources.AWS/AWSEKSDetector.cs	75.00%	2 Missing ⚠️
...enTelemetry.Resources.AWS/ResourceDetectorUtils.cs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4122      +/-   ##
==========================================
+ Coverage   72.77%   72.86%   +0.08%     
==========================================
  Files         469      452      -17     
  Lines       18272    17846     -426     
==========================================
- Hits        13298    13003     -295     
+ Misses       4974     4843     -131     

Flag	Coverage Δ
unittests-Instrumentation.Cassandra	?
unittests-Instrumentation.Remoting	?
unittests-Resources.AWS	73.88% <87.80%> (-0.80%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Resources.AWS/AWSECSDetector.cs	91.75% <100.00%> (+2.86%)	⬆️
...enTelemetry.Resources.AWS/ResourceDetectorUtils.cs	95.83% <94.73%> (-4.17%)	⬇️
src/OpenTelemetry.Resources.AWS/AWSEC2Detector.cs	73.52% <60.00%> (ø)
src/OpenTelemetry.Resources.AWS/AWSEKSDetector.cs	62.29% <75.00%> (ø)

... and 29 files with indirect coverage changes

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[Copilot]

Pull request overview

Updates the AWS resource detectors to reduce risk from unexpectedly large metadata service responses by limiting how much response body is consumed, while simplifying stream-reading code and removing sync-over-async helper usage.

Changes:

• Replace async HTTP metadata reads (and AsyncHelper) with a synchronous SendOutRequest path that uses shared HttpClientHelpers to cap response body reads.
• Simplify file/stream reading in ECS/EKS container-id and credential parsing.
• Wire HttpClientHelpers into the AWS project and document the change in the AWS changelog.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Resources.AWS.Tests/AsyncHelperTests.cs	Removes tests for AsyncHelper (helper is removed from production code).
src/OpenTelemetry.Resources.AWS/AsyncHelper.cs	Removes the sync-over-async helper now that call sites no longer need it.
src/OpenTelemetry.Resources.AWS/ResourceDetectorUtils.cs	Introduces synchronous metadata HTTP request helper and uses HttpClientHelpers for bounded response reads; stream/file helper simplifications.
src/OpenTelemetry.Resources.AWS/AWSEKSDetector.cs	Simplifies token/container-id file reads; switches to new synchronous request helper.
src/OpenTelemetry.Resources.AWS/AWSECSDetector.cs	Fixes/simplifies container-id file parsing; switches ECS metadata v4 fetches to new synchronous request helper.
src/OpenTelemetry.Resources.AWS/AWSEC2Detector.cs	Switches EC2 IMDS calls to new synchronous request helper and simplifies small helpers.
src/OpenTelemetry.Resources.AWS/OpenTelemetry.Resources.AWS.csproj	Links in src/Shared/HttpClientHelpers.cs for bounded response reading.
src/OpenTelemetry.Resources.AWS/CHANGELOG.md	Adds an Unreleased entry for limiting metadata HTTP response body consumption.

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