Alerting: add sync timer support (#114602)

- add new feature flag to support enabling the dispatcher sync timer on the alertmanager
- this attempts to synchronize the flushes across HA nodes to decrease amount of duplicate notifications

---------

Co-authored-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>

pkg/services/ngalert/notifier/dispatch_timer_test.go

@@ -0,0 +1,36 @@
+package notifier
+
+import (
+	"testing"
+
+	alertingNotify "github.com/grafana/alerting/notify"
+	"github.com/grafana/grafana/pkg/services/featuremgmt"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetDispatchTimer(t *testing.T) {
+	tests := []struct {
+		name             string
+		featureFlagValue bool
+		expected         alertingNotify.DispatchTimer
+	}{
+		{
+			name:             "feature flag enabled returns sync timer",
+			featureFlagValue: true,
+			expected:         alertingNotify.DispatchTimerSync,
+		},
+		{
+			name:             "feature flag disabled returns default timer",
+			featureFlagValue: false,
+			expected:         alertingNotify.DispatchTimerDefault,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			features := featuremgmt.WithFeatures(featuremgmt.FlagAlertingSyncDispatchTimer, tt.featureFlagValue)
+			result := GetDispatchTimer(features)
+			require.Equal(t, tt.expected, result)
+		})
+	}
+}