mapper: drop empty changes before fan-out

An empty change carries no work for any recipient, so it never becomes
a pending entry. Adds headscale_mapper_changes_dropped_total.

Updates #3417

(cherry picked from commit 286f1d5a12)

hscontrol/types/change/change.go

@@ -177,6 +177,11 @@ func (r Change) IsFull() bool {
 // This provides a bounded set of values suitable for Prometheus labels,
 // unlike [Change.Reason] which is free-form text for logging.
 func (r Change) Type() string {
+	// A suppressed update is a healthy outcome, not an unclassified one.
+	if r.IsEmpty() {
+		return "empty"
+	}
+
 	if r.IsFull() {
 		return "full"
 	}

hscontrol/types/change/change_test.go

@@ -561,9 +561,9 @@ func TestChange_Type(t *testing.T) {
 			want: "ping",
 		},
 		{
-			name:     "empty is unknown",
+			name:     "empty",
 			response: Change{},
-			want:     "unknown",
+			want:     "empty",
 		},
 	}