Fix duplicate TrackingIDs returned (#6960)

There is a small chance the newTrackingID() function in tracking.go
will return the same id to multiple simultaneous callers.
The function must return the value returned by atomic.AddUint64()
to be safe.

metric/tracking.go

@@ -34,8 +34,7 @@ var (
 )
 
 func newTrackingID() telegraf.TrackingID {
-	atomic.AddUint64(&lastID, 1)
-	return telegraf.TrackingID(lastID)
+	return telegraf.TrackingID(atomic.AddUint64(&lastID, 1))
 }
 
 func debugFinalizer(d *trackingData) {

metric/tracking_test.go

@@ -1,6 +1,7 @@
 package metric
 
 import (
+	"sync"
 	"testing"
 	"time"
 
@@ -30,6 +31,43 @@ func (d *deliveries) onDelivery(info telegraf.DeliveryInfo) {
 	d.Info[info.ID()] = info
 }
 
+func TestNewTrackingID(t *testing.T) {
+	var wg sync.WaitGroup
+	var a [100000]telegraf.TrackingID
+	var b [100000]telegraf.TrackingID
+
+	wg.Add(2)
+	go func() {
+		for i := 0; i < len(a); i++ {
+			a[i] = newTrackingID()
+		}
+		wg.Done()
+	}()
+	go func() {
+		for i := 0; i < len(b); i++ {
+			b[i] = newTrackingID()
+		}
+		wg.Done()
+	}()
+	wg.Wait()
+
+	// Find any duplicate TrackingIDs in arrays a and b. Arrays must be sorted in increasing order.
+	for i, j := 0, 0; i < len(a) && j < len(b); {
+		if a[i] == b[j] {
+			t.Errorf("Duplicate TrackingID: a[%d]==%d and b[%d]==%d.", i, a[i], j, b[j])
+			break
+		}
+		if a[i] > b[j] {
+			j++
+			continue
+		}
+		if a[i] < b[j] {
+			i++
+			continue
+		}
+	}
+}
+
 func TestTracking(t *testing.T) {
 	tests := []struct {
 		name      string