Follow-up after doing some more research out of curiosity: it looks like Fluent Bit's C library[1] may support OpenTelemetry trace output in future.
A quote from their v1.9.0 release notes[1]:
> We fully support Prometheus & OpenMetrics and we are also shipping experimental OpenTelemetry metrics support (spoiler: traces will come shortly!).
(I've no affiliation with Fluent nor particular knowledge of observability, so if anyone's interested in these findings, take them with a grain of salt :))
[1] - https://docs.fluentbit.io/manual/development/library_api