[cmd/telemetrygen] calculate and set span start and end times precisely

- previous approach depended on a second call of `time.Now()` which differed in value from the spans start time and introduced variance to the duration
- both start and end times are now set for both parent and child spans
- added test to ensure start and end times match the duration provided exactly

cmd/telemetrygen/internal/traces/worker.go

@@ -44,12 +44,18 @@ func (w worker) simulateTraces() {
 	limiter := rate.NewLimiter(w.limitPerSecond, 1)
 	var i int
 	for w.running.Load() {
+		spanStart := time.Now()
+		spanEnd := spanStart.Add(w.spanDuration)
+		endTimestampOption := trace.WithTimestamp(spanEnd)
+
 		ctx, sp := tracer.Start(context.Background(), "lets-go", trace.WithAttributes(
 			semconv.NetPeerIPKey.String(fakeIP),
 			semconv.PeerServiceKey.String("telemetrygen-server"),
 		),
 			trace.WithSpanKind(trace.SpanKindClient),
+			trace.WithTimestamp(spanStart),
 		)
+
 		for j := 0; j < w.loadSize; j++ {
 			sp.SetAttributes(attribute.String(fmt.Sprintf("load-%v", j), string(make([]byte, charactersPerMB))))
 		}
@@ -69,17 +75,17 @@ func (w worker) simulateTraces() {
 			semconv.PeerServiceKey.String("telemetrygen-client"),
 		),
 			trace.WithSpanKind(trace.SpanKindServer),
+			trace.WithTimestamp(spanStart),
 		)
 
 		if err := limiter.Wait(context.Background()); err != nil {
 			w.logger.Fatal("limiter waited failed, retry", zap.Error(err))
 		}
 
-		opt := trace.WithTimestamp(time.Now().Add(w.spanDuration))
 		child.SetStatus(w.statusCode, "")
-		child.End(opt)
+		child.End(endTimestampOption)
 		sp.SetStatus(w.statusCode, "")
-		sp.End(opt)
+		sp.End(endTimestampOption)
 
 		i++
 		if w.numTraces != 0 {

cmd/telemetrygen/internal/traces/worker_test.go

@@ -73,6 +73,38 @@ func TestRateOfSpans(t *testing.T) {
 	assert.True(t, len(syncer.spans) <= 20, "there should have been less than 20 spans, had %d", len(syncer.spans))
 }
 
+func TestSpanDuration(t *testing.T) {
+	// prepare
+	syncer := &mockSyncer{}
+
+	tracerProvider := sdktrace.NewTracerProvider()
+	sp := sdktrace.NewSimpleSpanProcessor(syncer)
+	tracerProvider.RegisterSpanProcessor(sp)
+	otel.SetTracerProvider(tracerProvider)
+
+	targetDuration := 1 * time.Second
+	cfg := &Config{
+		Config: common.Config{
+			Rate:          10,
+			TotalDuration: time.Second / 2,
+			WorkerCount:   1,
+		},
+		SpanDuration: targetDuration,
+	}
+
+	// sanity check
+	require.Len(t, syncer.spans, 0)
+
+	// test
+	require.NoError(t, Run(cfg, zap.NewNop()))
+
+	for _, span := range syncer.spans {
+		startTime, endTime := span.StartTime(), span.EndTime()
+		spanDuration := endTime.Sub(startTime)
+		assert.Equal(t, targetDuration, spanDuration)
+	}
+}
+
 func TestUnthrottled(t *testing.T) {
 	// prepare
 	syncer := &mockSyncer{}