Allow for all possible kinds of references

[kim]

This entails:

• an ActiveSpan is mutable, thus derefencing it as a ChildOf reference must occur at the end of it's lifetime (ie. we have to carry it around as a mutable reference)
• traced returns the FinishedSpan along with the result of the traced action, such that it can be used as a FollowsFrom reference in subsequent traced calls
• additionally, pure References may be used as obtained from the propagation carrier

[isovector]

I'm not entirely sure what the value of FollowsFrom is, but besides that everything looks good!

[kim]

I'm not entirely sure what the value of FollowsFrom is

I think it's when you don't want to wait for the parent span to complete only after all children have completed (see also: https://github.com/opentracing/opentracing-go/blob/master/tracer.go#L183-L207). Contrived example:

users <- traced (spanOpts "getUsers" mempty) $ const getUsers
async $ traced__ (spanOpts "cacheUserIds" (followsFrom (tracedSpan users))) $ const cacheUserIds (tracedResult users)
doSomethingElseWith (tracedResult users)

If the cache insert happens as a child, the users result is only available after that action has completed (and includes it's timing information).

Obviously, nothing prevents you at the moment to run children asynchronously and don't wait for their completion inside the traced block. I think the resulting trace would look the same.