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.

(cherry picked from commit 9823952)

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