Add finished flag

src/span.js

@@ -37,6 +37,7 @@ export default class Span {
     _tags: Array<Tag>;
     static _baggageHeaderCache: any;
     _references: Array<Reference>
+    _finished: boolean;
 
     constructor(tracer: any,
                 operationName: string,
@@ -54,6 +55,7 @@ export default class Span {
         this._references = references;
         this._logs = [];
         this._tags = [];
+        this._finished = false;
     }
 
     get firstInProcess(): boolean {
@@ -64,6 +66,10 @@ export default class Span {
         return this._operationName;
     }
 
+    get finished(): boolean {
+        return this._finished;
+    }
+
     static _getBaggageHeaderCache() {
         if (!Span._baggageHeaderCache) {
             Span._baggageHeaderCache = {};
@@ -154,11 +160,12 @@ export default class Span {
      * @param {number} finishTime - The time on which this span finished.
      **/
     finish(finishTime: ?number): void {
-        if (this._duration !== undefined) {
+        if (this.finished) {
             throw new Error('You can only call finish() on a span once.');
         }
 
         if (this._spanContext.isSampled()) {
+            this._finished = true;
             let endTime = finishTime || Utils.getTimestampMicros();
             this._duration = endTime - this._startTime;
             this._tracer._report(this);