proposal: support github.com/riverqueue/river

[wijayaerick]

river is a background job library written in Go. It would be great to add a native integration for this library.

River provides JobInsertMiddleware and WorkerMiddleware which can be used to inject/extract tracing data.

If this proposal is accepted, I'd be happy to contribute. I've created issue in the river repository as well (riverqueue/river#709) which include initial draft for the implementation. One thing I'm not quite sure is what data should be included in the span tags; I think we could refer from delayed_job from dd-trace-rb as it's also a background job library.