Make the end timestamp and mutable span as requirements in onending

[dmathieu]

Changes

OnEnding specifies that the ending timestamp should be set when the callback runs, and the span should be mutable. These are requirements, so it seems like they should be marked as so.

[MrAlias]

There is no way to query the end timestamp on the span. Does it matter if it is set or not?

[dmathieu]

It does, because if the callback duration will affect the timestamp value if we set it afterwards.

[MrAlias]

I see. Should it be clarified that the end time should exclude the time it takes to run the processors? It seems like requiring it being set is more restrictive than it needs to be.

[dmathieu]

That makes sense. I've made the change.