feat: Add OTEL compatible telemetry object builder

[askpt]

This PR

This pull request introduces a feature for generating telemetry events to track feature flag evaluations. It adds a new EvaluationEvent class, a builder for creating these events, constants for telemetry attributes, and metadata definitions. It also includes comprehensive unit tests to ensure the correctness of the implementation.

New Feature: Telemetry Event Generation

• EvaluationEvent class: Added a new class to represent evaluation events for feature flags, including properties for the event name and attributes. (src/OpenFeature/Telemetry/EvaluationEvent.cs)
• EvaluationEventBuilder class: Introduced a static builder class to construct EvaluationEvent instances using flag evaluation details and hook context. (src/OpenFeature/Telemetry/EvaluationEventBuilder.cs)

Supporting Components

• Telemetry constants: Defined a set of constants in TelemetryConstants to standardize attribute keys for OpenTelemetry-compliant feature flag events. (src/OpenFeature/Telemetry/TelemetryConstants.cs)
• Flag metadata attributes: Added TelemetryFlagMetadata to define well-known metadata attributes for telemetry events, such as contextId, flagSetId, and version. (src/OpenFeature/Telemetry/TelemetryFlagMetadata.cs)

Unit Tests

• EvaluationEventBuilderTests: Added unit tests to validate the behavior of EvaluationEventBuilder, including scenarios for handling errors, missing metadata, and missing attributes. (test/OpenFeature.Tests/Telemetry/EvaluationEventBuilderTests.cs)

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 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.08%. Comparing base (48f57cd) to head (b7d34fb).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/OpenFeature/Telemetry/EvaluationEventBuilder.cs	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #397      +/-   ##
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+ Coverage   86.87%   87.08%   +0.20%     
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  Files          43       45       +2     
  Lines        1722     1757      +35     
  Branches      180      184       +4     
==========================================
+ Hits         1496     1530      +34     
  Misses        187      187              
- Partials       39       40       +1     

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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]

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]

Hey @askpt, all the upstream blockers have been resolved. I've left some feedback on what you have so far. We should be able to get this in as soon as those issues are addressed. Thanks!