feat: Add OTEL compatible telemetry object builder

[askpt]

This PR

This pull request introduces a new feature for telemetry in the OpenFeature library, specifically focusing on evaluation events for feature flags. The main changes include the addition of new classes and constants to support the creation and handling of these events, as well as tests to ensure the functionality works as expected.

New Features for Telemetry:

• src/OpenFeature/Telemetry/EvaluationEvent.cs: Added the EvaluationEvent class to represent an evaluation event for feature flags, including properties for the event name, attributes, and body.
• src/OpenFeature/Telemetry/EvaluationEventBuilder.cs: Introduced the EvaluationEventBuilder class to create evaluation events based on hook context and flag evaluation details.

Constants for Telemetry:

• src/OpenFeature/Telemetry/TelemetryConstants.cs: Defined constants for various attributes used in telemetry events, such as Key, ErrorCode, Variant, ContextId, ErrorMessage, Reason, Provider, FlagSetId, and Version.
• src/OpenFeature/Telemetry/TelemetryEvaluationData.cs: Added the TelemetryEvaluationData class for event-specific data, specifically the evaluated value of the feature flag.
• src/OpenFeature/Telemetry/TelemetryFlagMetadata.cs: Introduced the TelemetryFlagMetadata class for well-known flag metadata attributes, such as ContextId, FlagSetId, and Version.

Tests for Evaluation Event Builder:

• test/OpenFeature.Tests/Telemetry/EvaluationEventBuilderTests.cs: Added unit tests for the EvaluationEventBuilder to verify that the evaluation events are built correctly, including handling of error details and missing variants.

Related Issues

Fixes #381

[Copilot]

PR Overview

This PR adds OTEL-compatible telemetry support to the OpenFeature library by introducing new classes, constants, and tests for building evaluation events used in feature flag evaluations.

• Added telemetry constants and flag metadata classes.
• Implemented an evaluation event builder to construct OTEL-compliant evaluation events.
• Included unit tests to verify the correct construction of evaluation events.

Reviewed Changes

File	Description
src/OpenFeature/Telemetry/TelemetryConstants.cs	Introduces constants for telemetry event attributes.
src/OpenFeature/Telemetry/EvaluationEventBuilder.cs	Implements the builder for evaluation events.
test/OpenFeature.Tests/Telemetry/EvaluationEventBuilderTests.cs	Adds tests to validate evaluation event building, including error handling and missing variants.
src/OpenFeature/Telemetry/TelemetryFlagMetadata.cs	Defines well-known metadata keys for telemetry.
src/OpenFeature/Telemetry/TelemetryEvaluationData.cs	Adds evaluation data constants for telemetry events.
src/OpenFeature/Telemetry/EvaluationEvent.cs	Provides the EvaluationEvent class used to represent an event.

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

src/OpenFeature/Telemetry/EvaluationEventBuilder.cs:28

• The reference 'Reason.Unknown' is undefined in this context; please ensure that the appropriate constant or enum is referenced instead.

attributes[TelemetryConstants.Reason] = !string.IsNullOrWhiteSpace(details.Reason) ? details.Reason?.ToLowerInvariant() : Reason.Unknown;

test/OpenFeature.Tests/Telemetry/EvaluationEventBuilderTests.cs:2

• The using directive 'using OpenFeature.Constant;' in the test file does not match the namespace of TelemetryConstants. Please update it to 'using OpenFeature.Telemetry;' to ensure consistency.

using OpenFeature.Constant;

[codecov]

Codecov Report

Attention: Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.36%. Comparing base (0821b3b) to head (6b48487).

Files with missing lines	Patch %	Lines
...rc/OpenFeature/Telemetry/EvaluationEventBuilder.cs	72.72%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #397      +/-   ##
==========================================
- Coverage   86.47%   86.36%   -0.11%     
==========================================
  Files          42       44       +2     
  Lines        1671     1702      +31     
  Branches      177      185       +8     
==========================================
+ Hits         1445     1470      +25     
  Misses        187      187              
- Partials       39       45       +6     

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[Copilot]

PR Overview

This PR adds support for OTEL-compatible telemetry events for feature flags in the OpenFeature library. The changes include the creation of new classes for evaluation events and their builders, the definition of related constants, and comprehensive tests for the new functionality.

• Added telemetry constants and flag metadata definitions.
• Introduced the EvaluationEvent and EvaluationEventBuilder classes.
• Added unit tests to verify the behavior of evaluation event creation.

Reviewed Changes

File	Description
src/OpenFeature/Telemetry/TelemetryConstants.cs	Defines constants for telemetry attribute keys.
src/OpenFeature/Telemetry/EvaluationEventBuilder.cs	Provides a builder for creating evaluation events; review notes below.
test/OpenFeature.Tests/Telemetry/EvaluationEventBuilderTests.cs	Contains tests that verify evaluation event creation behavior.
src/OpenFeature/Telemetry/TelemetryEvaluationData.cs	Defines additional telemetry event data constants.
src/OpenFeature/Telemetry/TelemetryFlagMetadata.cs	Specifies well-known flag metadata attribute keys.
src/OpenFeature/Telemetry/EvaluationEvent.cs	Implements the evaluation event model with attributes and body.

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/OpenFeature/Telemetry/EvaluationEventBuilder.cs:2

• The using directive 'OpenFeature.Constant' appears inconsistent with the location of TelemetryConstants, which is in the 'OpenFeature.Telemetry' namespace. Consider updating or removing this directive to avoid potential namespace conflicts.

using OpenFeature.Constant;

src/OpenFeature/Telemetry/EvaluationEventBuilder.cs:31

• The use of 'Reason.Unknown' appears to reference an undefined member. Consider defining a constant (e.g., TelemetryConstants.DefaultReason or similar) or replacing it with an appropriate default value such as "unknown".

attributes[TelemetryConstants.Reason] = !string.IsNullOrWhiteSpace(details.Reason) ? details.Reason?.ToLowerInvariant() : Reason.Unknown;

[beeme1mr]

Looks good but please make sure error code and reason is lowercase.

[beeme1mr]

This should be lowercase.

[askpt]

Thanks for reminding me. Should be fixed with 0d1a869

[beeme1mr]

FYI @askpt, I found out yesterday that the OTel team has required some changes to the semcon. Let's wait on merging until we have further clarity.

[askpt]

FYI @askpt, I found out yesterday that the OTel team has required some changes to the semcon. Let's wait on merging until we have further clarity.

@beeme1mr This PR is now updated with the latest semcov conventions. When we get the spec updated, we can merge this