setError() and sendError() don't work from inside error handler in Fastify

[luismiramirez]

The error-handling helpers from our library don't behave as expected when used inside Fastify error handler middleware.

Calling the OpenTelemetry API directly works. Using recordException() on the active span.

Internal link

[luismiramirez]

I couldn't reproduce using setErrorHandler() in Fastify 4 and 5. Both helpers setError() and sendError() work just fine. However, as the customer reported, they stop working when you set up a preHandler().

Here's a related issue from OpenTelemetry's official plugin, which is exactly the problem the customer is having. -> open-telemetry/opentelemetry-js-contrib#2355