[mcollina]

Summary

Fixes #196.

When an instrumented handler or hook throws, the same error was recorded as an exception event on both the child span where it was thrown and the parent request span (via the request's onError hook). Backends that map each exception event to an error occurrence (e.g. Elastic APM) counted a single failure twice.

Changes

• Track errors already recorded on a child (handler/hook) span in a Set scoped to the request.
• In the request span's recordErrorInSpanHook, skip span.recordException when the propagated error was already recorded on a child span, while still marking the request span as ERROR with the error message.
• Errors that never pass through a wrapped handler/hook — such as reply.send(err) or done(err) — are still recorded on the request span as before.
• The per-request tracking also handles primitive throws (e.g. throw 'foo') and does not leak state across requests.

Tests

Added tests covering:

• A thrown handler error produces one exception event (on the handler span), not on the request span.
• A thrown hook error produces one exception event (on the hook span), not on the request span.
• An error sent via reply.send(err) is still recorded on the request span.
• A primitive throw is recorded once.

All tests pass for Fastify v4 and v5 with 100% coverage, TypeScript checks, and lint.

[Tony133]

lgtm