add telemetry hook implementation guidelines

[beeme1mr]

This PR

• adds observability hook guidelines to the appendix

Notes

This is based on the updated telemetry hook @lukas-reining wrote. I tried to capture as much context as possible so that other maintainers can understand how the hook should be written.

Since this is in the appendix section, I decided to make the guide human-readable instead of just a list of requirements.

Follow-up Tasks

Create implementation issues for all SDKs based on these guidelines.

[gemini-code-assist]

Summary of Changes

Hello @beeme1mr, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces comprehensive guidelines for implementing observability hooks that emit OpenTelemetry signals during feature flag evaluations. The new guide, added to the specification's appendix, aims to standardize how maintainers write telemetry hooks across various SDKs by detailing signal emission patterns, hook lifecycle responsibilities, attribute transformations, and privacy considerations for flag values. This will facilitate consistent and robust telemetry integration.

Highlights

• New Telemetry Hook Implementation Guide: A comprehensive guide has been added to the observability appendix, detailing how to implement telemetry hooks for feature flag evaluations.
• Signal Emission Patterns: The guide outlines three distinct patterns for emitting OpenTelemetry signals: Span Events (recommended), Event Logging, and Standalone Spans, along with their advantages and disadvantages.
• Hook Lifecycle and Error Handling: Detailed instructions are provided for implementing the before, finally, and error stages of a hook, emphasizing graceful error handling that does not interrupt flag evaluation.
• Attribute Transformation and Privacy: Guidelines are included for transforming flag metadata into OpenTelemetry attributes and for handling sensitive flag values, with recommendations for redaction and configurable privacy options.
• Standardized Configuration and Patterns: Common configuration options and implementation patterns, such as a base class for shared functionality, are suggested to ensure consistency across SDK implementations.

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[gemini-code-assist]

Code Review

This pull request adds a comprehensive and well-structured guide for implementing telemetry hooks. The documentation covers important aspects like signal emission patterns, hook lifecycle, attribute transformations, privacy, configuration, and error handling. I've added a couple of suggestions to improve the clarity and precision of the guidelines, which is important for a specification document. Overall, this is a valuable addition.

[erka]

@beeme1mr Please clarify what the span event name should be - feature_flag.evaluation?

[beeme1mr]

@beeme1mr Please clarify what the span event name should be - feature_flag.evaluation?

@erka done!

[askpt]

LGTM! Just two minor questions in case they're typos.

[sahidvelji]

Does this include both error message and error type? Can we clarify that here?

[beeme1mr]

Okay, I'll try and clarify what I mean. Here's an example for JS.
https://github.com/open-feature/js-sdk-contrib/pull/1372/files#diff-8d1358ebed8b9039db57f7495ae547872e4f246efe0f198838e98a2d2208cd22R39-R41

[beeme1mr]

I couldn't think of a way to rephrase this to make it more clear. Do you have any recommendations? The goal of the setting it control whether or not the hook should capture exception via the OTel API.

[sahidvelji]

Ok I see. Would it make sense to include a link to this?
https://opentelemetry.io/docs/specs/semconv/exceptions/exceptions-logs/