feat(core): support object type attribute on spans (#496)

* feat(core): support object type attribute on spans

* docs(span): document JSON.stringify-able span.addAttribute value

* fix(opencensus-core): update no record span addAttribute params

* fix(core): no record span addAttribute args

packages/opencensus-core/src/trace/model/no-record/no-record-span.ts

@@ -149,7 +149,7 @@ export class NoRecordSpan implements types.Span {
   }
 
   /** No-op implementation of this method. */
-  addAttribute(key: string, value: string|number|boolean) {}
+  addAttribute(key: string, value: string|number|boolean|object) {}
 
   /** No-op implementation of this method. */
   addAnnotation(

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

@@ -193,9 +193,10 @@ export class Span implements types.Span {
   /**
    * Adds an atribute to the span.
    * @param key Describes the value added.
-   * @param value The result of an operation.
+   * @param value The result of an operation. If the value is a typeof object
+   *     it has to be JSON.stringify-able, cannot contain circular dependencies.
    */
-  addAttribute(key: string, value: string|number|boolean) {
+  addAttribute(key: string, value: string|number|boolean|object) {
     if (this.attributes[key]) {
       delete this.attributes[key];
     }
@@ -208,7 +209,9 @@ export class Span implements types.Span {
         delete this.attributes[attributeKeyToDelete];
       }
     }
-    this.attributes[key] = value;
+    const serializedValue =
+        typeof value === 'object' ? JSON.stringify(value) : value;
+    this.attributes[key] = serializedValue;
   }
 
   /**

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

@@ -406,7 +406,7 @@ export interface Span {
    * @param key Describes the value added.
    * @param value The result of an operation.
    */
-  addAttribute(key: string, value: string|number|boolean): void;
+  addAttribute(key: string, value: string|number|boolean|object): void;
 
   /**
    * Adds an annotation to the span.

packages/opencensus-core/test/test-span.ts

@@ -195,6 +195,10 @@ describe('Span', () => {
         assert.equal(
             span.attributes['testKey' + attType], 'testValue' + attType);
       });
+      span.addAttribute('object', {foo: 'bar'});
+      assert.equal(span.attributes['object'], '{"foo":"bar"}');
+      span.addAttribute('array', [1, 2, 3]);
+      assert.equal(span.attributes['array'], '[1,2,3]');
     });
 
     it('should drop extra attributes', () => {