Add class documentation FunctionSegmentNotDefinedErrorunexpected and make end_time optional (#845)

Author: ijemmy

packages/tracing/tests/helpers/FunctionSegmentNotDefinedError.ts

@@ -1,3 +1,10 @@
+/**
+ * Thrown when the function segement (AWS::Lambda::Function) is not found in a trace.
+ * 
+ * X-Ray segments are process asynchronously. They may not be available even after 
+ * the trace has already appeared. In that case, the function segment may be missing.
+ * We will throw this error to notify caller.
+ */
 export class FunctionSegmentNotDefinedError extends Error {
   public constructor(msg: string) {
     super(msg);

packages/tracing/tests/helpers/tracesUtils.ts

@@ -19,7 +19,7 @@ interface ParsedDocument {
   name: string
   id: string
   start_time: number
-  end_time: number
+  end_time?: number
   // This flag may be set if the segment hasn't been fully processed
   // The trace may have already appeared in the `getTraceSummaries` response 
   // but a segment may still be in_progress