feat(openresty): add status code tag expand helper

pkg/render/openresty/status_codes_test.go

@@ -0,0 +1,27 @@
+package openresty
+
+import "testing"
+
+func TestExpandStatusCodeTags(t *testing.T) {
+	t.Parallel()
+	codes, err := ExpandStatusCodeTags([]string{"500-502", "522", "501"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	// want sorted unique: 500,501,502,522
+	if len(codes) != 4 || codes[0] != 500 || codes[3] != 522 {
+		t.Fatalf("got %v", codes)
+	}
+	_, err = ExpandStatusCodeTags([]string{"399"})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	_, err = ExpandStatusCodeTags([]string{"503-500"})
+	if err == nil {
+		t.Fatal("expected reverse range error")
+	}
+	_, err = ExpandStatusCodeTags([]string{"5xx"})
+	if err == nil {
+		t.Fatal("expected syntax error")
+	}
+}