test: fix flaky TestTemporalityStartTimes by introducing a mockClock with added Time (#39306)

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#### Description

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#### Link to tracking issue
Fixes
#39219 (comment)

<!--Describe what testing was performed and which tests were added.-->
#### Testing

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#### Documentation

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Author: NickAnge

.chloggen/nickange_telemetrygen_metrics_fix-flaky-test.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: telemetrygen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix flaky test TestTemporalityStartTimes
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [39219]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

cmd/telemetrygen/pkg/metrics/clock.go

@@ -0,0 +1,27 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package metrics
+
+import (
+	"time"
+)
+
+type Clock interface {
+	Now() time.Time
+}
+
+type realClock struct{}
+
+func (c *realClock) Now() time.Time {
+	return time.Now()
+}
+
+type mockClock struct {
+	now time.Time
+}
+
+func (c *mockClock) Now() time.Time {
+	c.now = c.now.Add(100 * time.Millisecond) // Add 100ms to the mock clock to avoid timestamp collisions
+	return c.now
+}

cmd/telemetrygen/pkg/metrics/metrics.go

@@ -77,6 +77,7 @@ func run(c *Config, expF exporterFunc, logger *zap.Logger) error {
 			wg:                     &wg,
 			logger:                 logger.With(zap.Int("worker", i)),
 			index:                  i,
+			clock:                  &realClock{},
 		}
 		exp, err := expF()
 		if err != nil {

cmd/telemetrygen/pkg/metrics/worker.go

@@ -29,6 +29,7 @@ type worker struct {
 	wg                     *sync.WaitGroup              // notify when done
 	logger                 *zap.Logger                  // logger
 	index                  int                          // worker index
+	clock                  Clock                        // clock
 }
 
 // We use a 15-element bounds slice for histograms below, so there must be 16 buckets here.
@@ -84,13 +85,14 @@ var histogramBucketSamples = []struct {
 func (w worker) simulateMetrics(res *resource.Resource, exporter sdkmetric.Exporter, signalAttrs []attribute.KeyValue) {
 	limiter := rate.NewLimiter(w.limitPerSecond, 1)
 
-	startTime := time.Now()
+	startTime := w.clock.Now()
 
 	var i int64
 	for w.running.Load() {
 		var metrics []metricdata.Metrics
+		now := w.clock.Now()
 		if w.aggregationTemporality.AsTemporality() == metricdata.DeltaTemporality {
-			startTime = time.Now().Add(-1 * time.Second)
+			startTime = now.Add(-1 * time.Second)
 		}
 		switch w.metricType {
 		case MetricTypeGauge:
@@ -99,7 +101,7 @@ func (w worker) simulateMetrics(res *resource.Resource, exporter sdkmetric.Expor
 				Data: metricdata.Gauge[int64]{
 					DataPoints: []metricdata.DataPoint[int64]{
 						{
-							Time:       time.Now(),
+							Time:       now,
 							Value:      i,
 							Attributes: attribute.NewSet(signalAttrs...),
 							Exemplars:  w.exemplars,
@@ -116,7 +118,7 @@ func (w worker) simulateMetrics(res *resource.Resource, exporter sdkmetric.Expor
 					DataPoints: []metricdata.DataPoint[int64]{
 						{
 							StartTime:  startTime,
-							Time:       time.Now(),
+							Time:       now,
 							Value:      i,
 							Attributes: attribute.NewSet(signalAttrs...),
 							Exemplars:  w.exemplars,
@@ -139,7 +141,7 @@ func (w worker) simulateMetrics(res *resource.Resource, exporter sdkmetric.Expor
 					DataPoints: []metricdata.HistogramDataPoint[int64]{
 						{
 							StartTime:  startTime,
-							Time:       time.Now(),
+							Time:       now,
 							Attributes: attribute.NewSet(signalAttrs...),
 							Exemplars:  w.exemplars,
 							Count:      totalCount,

cmd/telemetrygen/pkg/metrics/worker_test.go

@@ -5,7 +5,6 @@ package metrics
 
 import (
 	"context"
-	"runtime"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -478,9 +477,6 @@ func configWithMultipleAttributes(metric MetricType, qty int) *Config {
 }
 
 func TestTemporalityStartTimes(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("http://github.com/open-telemetry/opentelemetry-collector-contrib/issues/39219")
-	}
 	tests := []struct {
 		name        string
 		temporality AggregationTemporality
@@ -490,23 +486,31 @@ func TestTemporalityStartTimes(t *testing.T) {
 			name:        "Cumulative temporality keeps same start timestamp",
 			temporality: AggregationTemporality(metricdata.CumulativeTemporality),
 			checkTimes: func(t *testing.T, firstTime, secondTime time.Time) {
-				assert.Equal(t, firstTime, secondTime,
-					"cumulative metrics should have same start time")
+				if !assert.Equal(t, firstTime, secondTime,
+					"cumulative metrics should have same start time") {
+					logTimestampDiff(t, firstTime, secondTime)
+				}
 			},
 		},
 		{
 			name:        "Delta temporality has different start timestamps",
 			temporality: AggregationTemporality(metricdata.DeltaTemporality),
 			checkTimes: func(t *testing.T, firstTime, secondTime time.Time) {
-				assert.True(t, secondTime.After(firstTime),
-					"delta metrics should have increasing start times")
+				if !assert.True(t, secondTime.After(firstTime),
+					"delta metrics should have increasing start times") {
+					logTimestampDiff(t, firstTime, secondTime)
+				}
 			},
 		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			m := &mockExporter{}
+			clock := &mockClock{
+				now: time.Now(),
+			}
+
 			running := &atomic.Bool{}
 			running.Store(true)
 
@@ -522,6 +526,7 @@ func TestTemporalityStartTimes(t *testing.T) {
 				limitPerSecond:         rate.Inf,
 				logger:                 zap.NewNop(),
 				wg:                     wg,
+				clock:                  clock,
 			}
 
 			w.simulateMetrics(resource.Default(), m, nil)
@@ -540,3 +545,13 @@ func TestTemporalityStartTimes(t *testing.T) {
 		})
 	}
 }
+
+func logTimestampDiff(t *testing.T, firstTime, secondTime time.Time) {
+	t.Logf("Timestamp debug logging:\n"+
+		"First start time:  %s\n"+
+		"Second start time: %s\n"+
+		"Difference: %v",
+		firstTime.String(),
+		secondTime.String(),
+		secondTime.Sub(firstTime))
+}