Log consumers should be able to differentiate sampled from unsampled traces

What happened?

Log lines inside a traced operation contain a traceId MDC key. Tracing output may be loaded by ID in our logging pipeline, however most traceIds are not sampled, and as such may not be viewed. It's difficult to tell which traceIds one can search for.

What did you want to happen?

It would be helpful if we set a second MDC key-value pair to differentiate sampled and unsampled traces.

Proposal

Tracer should set sampled:true when setting the traceId key for a sampled request.

MDC key name: sampled, traceSampled, better ideas? sampled is simple and easy to remember.

Should we set sampled:false for traces that are not sampled? This would allow us to differentiate versions which have not yet upgraded, however it would also slightly increase the overhead of every log line. While it may be helpful today, I don't think it's necessary long term.