fix start time of span is zero issue

skywalking-application-toolkit/skywalking-toolkit-opentracing/src/main/java/com/a/eye/skywalking/toolkit/opentracing/SkyWalkingSpanBuilder.java

@@ -94,6 +94,9 @@ public class SkyWalkingSpanBuilder implements Tracer.SpanBuilder {
 
     @Override
     public Span start() {
+        if (startTime == 0){
+            startTime = System.currentTimeMillis();
+        }
         return new SkyWalkingSpan(this.operationName, this.startTime, this.tags);
     }