[api/tracing] Add missing edge to exit event
[api/tracing] Add missing edge to exit event

Previously this was resulting in broken traces, since only the abstract
edge was being reported. Now the exit event is causally tied to the last
event that was reported from this layer, which may or may not be the
entry event.

appneta/ruby-traceview,boyang-li/oboe-ruby,appneta/ruby-traceview,tlunter/oboe-ruby,boyang-li/oboe-ruby,tlunter/oboe-ruby,appneta/ruby-traceview,tlunter/oboe-ruby,boyang-li/oboe-ruby,boyang-li/oboe-ruby,appneta/ruby-traceview,tlunter/oboe-ruby