[OpenTelemetry] Handle duplicate sampler attributes

[xiang17]

Fixes Azure/azure-sdk-for-net#58948
Design discussion issue #

Changes

Replaced ActivityTagsCollection.Add with indexer assignment when applying sampler attributes during Activity creation.

This prevents Activity creation from throwing when multiple matching TracerProvider instances return an attribute with the same key. The behavior now matches the existing legacy Activity path, which applies sampler attributes using Activity.SetTag replacement semantics.

When multiple samplers return different values for the same key, the value from the last invoked listener is retained.

Added a regression test with two providers that return the same attribute key.

Motivation

OpenTelemetry supports multiple TracerProvider instances, and .NET ActivitySource invokes every matching ActivityListener while creating one Activity. These listeners contribute to the Activity's shared sampling-tag collection.

Previously, the second provider failed while adding a duplicate key:

System.InvalidOperationException: The collection already contains item with same key 'duplicate-sampler-attribute'

This exception prevents Activity creation and, in ASP.NET Core TestHost, may present as a request timeout.

Validation

Before the fix, the new regression test failed in TracerProviderSdk.ComputeActivitySamplingResult.

After the fix:

• TracerProviderSdkDuplicateSamplerAttributesUseLastValue passed.
• Full TracerProviderSdkTests class passed: 67 tests.
• Production source formatting verification passed.

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)

[opentelemetry-pr-dashboard]

Pull request dashboard status

Merged · refreshed 2026-07-29 06:33 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (9440662) to head (f081445).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7558      +/-   ##
==========================================
- Coverage   90.43%   90.39%   -0.04%     
==========================================
  Files         290      290              
  Lines       15989    15989              
==========================================
- Hits        14459    14453       -6     
- Misses       1530     1536       +6     

Flag	Coverage Δ
unittests-Project-Experimental	90.35% <100.00%> (-0.17%)	⬇️
unittests-Project-Stable	90.43% <100.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
src/OpenTelemetry/Trace/TracerProviderSdk.cs	98.72% <100.00%> (ø)

... and 3 files with indirect coverage changes

[cijothomas]

LGTM.