Test metric aggregate times (#5323)

sdk/metric/internal/aggregate/aggregate_test.go

@@ -6,6 +6,7 @@ package aggregate // import "go.opentelemetry.io/otel/sdk/metric/internal/aggreg
 import (
 	"context"
 	"strconv"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -39,16 +40,39 @@ var (
 	fltrBob   = attribute.NewSet(userBob)
 
 	// Sat Jan 01 2000 00:00:00 GMT+0000.
-	staticTime    = time.Unix(946684800, 0)
-	staticNowFunc = func() time.Time { return staticTime }
-	// Pass to t.Cleanup to override the now function with staticNowFunc and
-	// revert once the test completes. E.g. t.Cleanup(mockTime(now)).
-	mockTime = func(orig func() time.Time) (cleanup func()) {
-		now = staticNowFunc
-		return func() { now = orig }
-	}
+	y2k = time.Unix(946684800, 0)
 )
 
+// y2kPlus returns the timestamp at n seconds past Sat Jan 01 2000 00:00:00 GMT+0000.
+func y2kPlus(n int64) time.Time {
+	d := time.Duration(n) * time.Second
+	return y2k.Add(d)
+}
+
+// clock is a test clock. It provides a predictable value for now() that can be
+// reset.
+type clock struct {
+	ticks atomic.Int64
+}
+
+// Now returns the mocked time starting at y2kPlus(0). Each call to Now will
+// increment the returned value by one second.
+func (c *clock) Now() time.Time {
+	old := c.ticks.Add(1) - 1
+	return y2kPlus(old)
+}
+
+// Reset resets the clock c to tick from y2kPlus(0).
+func (c *clock) Reset() { c.ticks.Store(0) }
+
+// Register registers clock c's Now method as the now var. It returns an
+// unregister func that should be called to restore the original now value.
+func (c *clock) Register() (unregister func()) {
+	orig := now
+	now = c.Now
+	return func() { now = orig }
+}
+
 func dropExemplars[N int64 | float64]() exemplar.Reservoir {
 	return exemplar.Drop()
 }