fix: set currentRootSpan before start and set parentSpanId at span constructor (#32)

* fix: set currentRootSpan before start, set parentSpanId at span constructor
     and add options to SpanBase.spanContext

* refactor: changes to address review comments

Author: fabiogomessilva

packages/opencensus-core/src/trace/model/span-base.ts

@@ -116,10 +116,10 @@ export abstract class SpanBase implements types.Span {
   /** Gives the TraceContext of the span. */
   get spanContext(): types.SpanContext {
     return {
-      traceId: this.traceId.toString(),
-      spanId: this.id.toString(),
-      parentSpanId: this.parentSpanId
-    } as types.SpanContext;
+      traceId: this.traceId,
+      spanId: this.id,
+      options: 0x1  // always traced
+    };
   }
 
   /**

packages/opencensus-core/src/trace/model/span.ts

@@ -32,22 +32,14 @@ export class Span extends SpanBase implements types.Span {
     super();
     this.root = root;
     this.logger = this.root.logger || logger.logger();
+    this.parentSpanId = root.id;
   }
 
   /** Gets trace id of span. */
   get traceId(): string {
     return this.root.traceId;
   }
 
-  /** Gets trace context of span. */
-  get spanContext(): types.SpanContext {
-    return {
-      traceId: this.traceId.toString(),
-      spanId: this.id.toString(),
-      options: 1  // always traced
-    };
-  }
-
   /** Starts the span instance. */
   start() {
     super.start();
@@ -57,7 +49,6 @@ export class Span extends SpanBase implements types.Span {
   }
 
 
-
   /** Ends the span. */
   end(): void {
     super.end();

packages/opencensus-core/src/trace/model/tracer.ts

@@ -126,8 +126,8 @@ export class Tracer implements types.Tracer {
             this.sampler.shouldSample(aRoot.traceId);
 
         if (sampleDecision) {
-          aRoot.start();
           this.currentRootSpan = aRoot;
+          aRoot.start();
           newRoot = aRoot;
         }
       } else {