add example for use of traces

[tecknicaltom]

It would be beneficial to add an example of how the traces feature is expected to be used.

I'm currently attempting to add tracing with otel to a codebase that is configured with slogdriver. I understand that slogdriver is attempting to be library agnostic with tracing data, but it looks like it's currently written with the intent that every place in code that creates a trace or adds a new span would have to interact with slogdriver to add a slogdriver.Trace to the context. This doesn't seem ideal. Would it instead be useful to be able to provide slogdriver.NewHandler() a function in the config that encapsulates retrieving the trace context from the current context?