Add and update comments in ui/metrics/event.go

The event.go code was refactored to follow the best practices
of golang code style.

Bug: b/172918681
Test: go test ./ui/metrics
Change-Id: Ic57d7cd576a9b146530ae37f03f4f5679d6cffef

ui/metrics/event_test.go

@@ -28,14 +28,14 @@ func TestEnd(t *testing.T) {
 	_now = func() time.Time { return startTime.Add(dur) }
 	defer func() { _now = initialNow }()
 
-	eventTracer := &eventTracerImpl{}
-	eventTracer.activeEvents = append(eventTracer.activeEvents, event{
+	et := &EventTracer{}
+	et.push(&event{
 		desc:  "test",
 		name:  "test",
 		start: startTime,
 	})
 
-	perf := eventTracer.End(tracer.Thread(0))
+	perf := et.End(tracer.Thread(0))
 	if perf.GetRealTime() != uint64(dur.Nanoseconds()) {
 		t.Errorf("got %d, want %d nanoseconds for event duration", perf.GetRealTime(), dur.Nanoseconds())
 	}