PendingTrace: add a TODO to move time fields into DDTracer

dd-trace-ot/src/main/java/datadog/opentracing/PendingTrace.java

@@ -30,6 +30,7 @@ public class PendingTrace extends ConcurrentLinkedDeque<DDSpan> {
   private final DDTracer tracer;
   private final long traceId;
 
+  // TODO: consider moving these time fields into DDTracer to ensure that traces have precise relative time
   /** Trace start time in nano seconds measured up to a millisecond accuracy */
   private final long startTimeNano;
   /** Nano second ticks value at trace start */