Add OpenTelemetry span events and missing instrumentation

[dahlia]

Summary

Enhanced OpenTelemetry observability by adding span events to capture rich activity data and instrumenting previously uncovered operations.

This PR implements the alternative approach proposed in #323, leveraging existing OpenTelemetry infrastructure instead of creating a new FederationObserver interface.

Changes

Span events added

Span events now record complete activity JSON payloads and verification status, enabling richer observability without relying solely on span attributes (which only support primitive values):

• activitypub.activity.received in the activitypub.inbox span

  • Records full activity JSON
  • Verification status (activity verified, HTTP signatures verified, LD signatures verified)
  • Actor information

• activitypub.activity.sent in the activitypub.send_activity span

  • Records activity JSON
  • Target inbox URL

• activitypub.object.fetched in the activitypub.lookup_object span

  • Records object type
  • Complete JSON-LD representation

New spans added

• docloader.fetch for document loader operations

  • Tracks URL fetching
  • HTTP redirects
  • Final document URLs

• activitypub.verify_key_ownership for key ownership verification

  • Records actor ID, key ID
  • Verification result and method used

Test coverage

Added comprehensive test coverage using a new TestSpanExporter helper in packages/fedify/src/testing/otel.ts to verify all instrumentation is properly recorded:

• Inbox handler span events
• Outbound activity sending span events
• Object lookup span events
• Document loader spans (including redirect handling)
• Key ownership verification spans

Documentation

Updated docs/manual/opentelemetry.md with:

• New spans and their attributes
• Span events section with detailed attribute documentation
• Example FedifyDebugExporter implementation showing how to build custom observability tools

Use cases

This enhancement enables developers to:

1. Build custom debugging dashboards by implementing custom SpanExporters that process span events
2. Inspect full activity payloads without modifying Fedify's core logging
3. Track verification failures with detailed context about what was verified and how
4. Monitor federation performance with detailed document fetching metrics

Breaking changes

None. This is a backward-compatible enhancement that adds new observability data points.

Resolves #323

[gemini-code-assist[bot]]

Summary of Changes

Hello @dahlia, I'm Gemini Code Assist[^1]! 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 significantly enhances the observability of the Fedify project by integrating more comprehensive OpenTelemetry tracing. It introduces detailed span events to capture rich, structured data like full activity payloads and verification statuses, which was previously not possible with standard span attributes. Additionally, new spans have been added to cover critical operations such as document loading and cryptographic key ownership verification, providing deeper insights into the system's behavior and performance. These enhancements empower developers to build more sophisticated debugging and monitoring tools.

Highlights

• Enhanced OpenTelemetry Instrumentation with Span Events: Detailed activity JSON payloads and verification statuses are now captured as span events, providing richer observability than traditional span attributes which are limited to primitive values. This includes events for received activities, sent activities, and fetched objects.
• New OpenTelemetry Spans for Key Operations: Introduced new spans for previously uninstrumented operations: docloader.fetch to track URL fetching and redirects, and activitypub.verify_key_ownership to monitor cryptographic key ownership verification processes.
• Comprehensive Test Coverage for Observability: Added extensive test coverage for the new instrumentation using a custom TestSpanExporter, ensuring that all new spans and events are properly recorded and verifiable.
• Updated OpenTelemetry Documentation: The docs/manual/opentelemetry.md file has been updated to reflect the new spans and span events, including detailed attribute documentation and an example FedifyDebugExporter to guide developers in building custom observability tools.

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