Merge oc_span and oc_trace

[hauleth]

All functions in oc_span are already present in the oc_trace. This will reduce public API of the library (good) but will slightly increase complexity of the oc_trace module (bad).

[tsloughter]

oc_span is the functional interface to a span. oc_trace modifies an active span in the span store.

[essen]

Both modules have an identical link/4 function oddly enough.

[hauleth]

@tsloughter in that case oc_trace should land "under" the oc_span, for example as oc_span_trace or something like that. Otherwise it can be confusing to the users about the difference between trace and span.

[tsloughter]

Yea, the extra link/4 is a mistake and can be removed from either of them.

I think we can fix this with docs in the modules to explain in oc_trace this is the module for interacting with your traces and that oc_span is really more of a private module, only to be used if you are writing something lower level, not instrumenting your own code.

Or actually marking it private so no docs are given for it.

Maybe there is also a better name for oc_trace but I don't think oc_span_trace is it.