Notifier: pass parameters to goroutine explicitly

Avoids possible false sharing between loops.

Plausibly there is no problem in the current code, but it's easy enough to write it more safely.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

notifier/notifier.go

@@ -539,9 +539,9 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 			ctx, cancel := context.WithTimeout(n.ctx, time.Duration(ams.cfg.Timeout))
 			defer cancel()
 
-			go func(client *http.Client, url string) {
+			go func(ctx context.Context, client *http.Client, url string, payload []byte, count int) {
 				if err := n.sendOne(ctx, client, url, payload); err != nil {
-					level.Error(n.logger).Log("alertmanager", url, "count", len(amAlerts), "msg", "Error sending alert", "err", err)
+					level.Error(n.logger).Log("alertmanager", url, "count", count, "msg", "Error sending alert", "err", err)
 					n.metrics.errors.WithLabelValues(url).Inc()
 				} else {
 					numSuccess.Inc()
@@ -550,7 +550,7 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 				n.metrics.sent.WithLabelValues(url).Add(float64(len(amAlerts)))
 
 				wg.Done()
-			}(ams.client, am.url().String())
+			}(ctx, ams.client, am.url().String(), payload, len(amAlerts))
 		}
 
 		ams.mtx.RUnlock()