Attend to review comments

CHANGELOG.md

@@ -3,6 +3,7 @@
 [Full changelog](https://github.com/mozilla/glean.js/compare/v0.10.2...main)
 
 * [#202](https://github.com/mozilla/glean.js/pull/202): Add a testing API for the ping type.
+* [#253](https://github.com/mozilla/glean.js/pull/253): Implement the timespan metric type.
 
 # v0.10.2 (2021-04-26)
 

glean/src/core/metrics/types/timespan.ts

@@ -166,7 +166,10 @@ class TimespanMetricType extends MetricType {
    * No error is recorded if no `start()` was called.
    */
   cancel(): void {
-    this.startTime = undefined;
+    Context.dispatcher.launch(() => {
+      this.startTime = undefined;
+      return Promise.resolve();
+    });
   }
 
   /**
@@ -190,6 +193,7 @@ class TimespanMetricType extends MetricType {
     });
 
     if (value) {
+      // `payload` will truncate to the defined time_unit at the time of recording.
       return (new TimespanMetric(value)).payload();
     }
   }

glean/tests/core/metrics/timespan.spec.ts

@@ -210,7 +210,7 @@ describe("TimespanMetric", function() {
     assert.deepStrictEqual(consoleErrorSpy.callCount, 1);
   });
 
-  it("cancel does not store", async function() {
+  it("cancel does not store and clears start time", async function() {
     const metric = new TimespanMetricType({
       category: "aCategory",
       name: "aTimespan",
@@ -222,6 +222,7 @@ describe("TimespanMetric", function() {
     metric.start();
     metric.cancel();
     assert.strictEqual(await metric.testGetValue("aPing"), undefined);
+    assert.strictEqual(metric["startTime"], undefined);
   });
 
   it("nothing is stored before stop", async function() {