avoid race (thanks andrew)

ddtrace/tracer/span.go

@@ -497,12 +497,12 @@ func (s *span) finish(finishTime int64) {
 		// a single kept span keeps the whole trace.
 		s.context.trace.keep()
 	}
-	s.context.finish()
 	if log.DebugEnabled() {
 		// avoid allocating the ...interface{} argument if debug logging is disabled
 		log.Debug("Finished Span: %v, Operation: %s, Resource: %s, Tags: %v, %v",
 			s, s.Name, s.Resource, s.Meta, s.Metrics)
 	}
+	s.context.finish()
 }
 
 // newAggregableSpan creates a new summary for the span s, within an application