Implement processor for journald to opentelemetry semantic mapping

Is your feature request related to a problem? Please describe. Journald implements structured fields with certain semantic. Opentelemetry has their semantic convention with similar fields.

https://github.com/open-telemetry/opentelemetry-specification/pull/4995 tries to describe the mapping between these in the opentelemetry appendix.

Describe the solution you'd like

I would like to propose a built in processor that would convert from a jorunald input (systemd) to an opentelemetry spec compliant envelope with the semantic of the fields preserved.

Describe alternatives you've considered

• I tried to implement the mapping by combining the existing processorts (rename, etc.) but they don't allow all the required operataions-

• I also tried to implement this as lua and WASM plugin. This works but it adds additional dependencies to fluentbit.

• Last I also implemented it as a processor in C, this works, see https://github.com/bachp/fluent-bit/tree/journald-mapping

Additional context

• Currently I have an out of tree version of this plugin that I can keep using if this is not desired by upstream.
• The main question to answer is if this processor is something that should live upstream in fluent-bit or external.