fix(internal/librarian/golang): skip updating snippet metadata in nested module (#4115)

Skip updating snippet metadata in nested module because it belongs to
another Go library.

For #3617

Author: JoeWang1127

internal/librarian/golang/generate.go

@@ -363,6 +363,7 @@ func generateREADME(library *config.Library, api *serviceconfig.API, moduleRoot
 }
 
 // updateSnippetMetadata updates the snippet metadata files with the correct library version.
+// Skip nested module if exists.
 func updateSnippetMetadata(library *config.Library, output string) error {
 	baseDir := filepath.Join(output, "internal", "generated", "snippets", library.Name)
 	return filepath.WalkDir(baseDir, func(path string, d fs.DirEntry, err error) error {
@@ -373,7 +374,13 @@ func updateSnippetMetadata(library *config.Library, output string) error {
 			}
 			return err
 		}
-		if d.IsDir() || !strings.HasPrefix(d.Name(), "snippet_metadata") {
+		if d.IsDir() {
+			if library.Go != nil && d.Name() == library.Go.NestedModule {
+				return fs.SkipDir
+			}
+			return nil
+		}
+		if !strings.HasPrefix(d.Name(), "snippet_metadata") {
 			return nil
 		}
 		read, err := os.ReadFile(path)

internal/librarian/golang/generate_test.go

@@ -419,6 +419,90 @@ func TestUpdateSnippetMetadata(t *testing.T) {
 	}
 }
 
+func TestUpdateSnippetMetadata_Skipped(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		library  *config.Library
+		dir      string
+		fileName string
+		data     string
+	}{
+		{
+			name: "skipped due to nested module",
+			library: &config.Library{
+				Name:    "bigquery",
+				Version: "1.2.3",
+				Go: &config.GoModule{
+					NestedModule: "v2",
+				},
+			},
+			dir:      filepath.Join("internal", "generated", "snippets", "bigquery", "v2", "apiv2"),
+			fileName: "snippet_metadata.google.cloud.bigquery.v2.json",
+			data: `{ 
+ "clientLibrary": {
+    "name": "cloud.google.com/go/bigquery/v2/apiv2",
+    "version": "$VERSION",
+    "language": "GO",
+    "apis": [
+      {
+        "id": "google.cloud.bigquery.v2",
+        "version": "v2"
+      }
+    ]
+ }
+}
+`,
+		},
+		{
+			name: "skipped due to non-snippets file name",
+			library: &config.Library{
+				Name:    "bigquery",
+				Version: "1.2.3",
+			},
+			dir:      filepath.Join("internal", "generated", "snippets", "bigquery", "apiv1"),
+			fileName: "non_metadata.google.cloud.bigquery.v1.json",
+			data: `{ 
+ "clientLibrary": {
+    "name": "cloud.google.com/go/bigquery/apiv1",
+    "version": "$VERSION",
+    "language": "GO",
+    "apis": [
+      {
+        "id": "google.cloud.bigquery.v1",
+        "version": "v1"
+      }
+    ]
+ }
+}
+`,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			err := os.MkdirAll(filepath.Join(tmpDir, test.dir), 0755)
+			if err != nil {
+				t.Fatal(err)
+			}
+			metadataFile := filepath.Join(tmpDir, test.dir, test.fileName)
+			if err := os.WriteFile(metadataFile, []byte(test.data), 0755); err != nil {
+				t.Fatal(err)
+			}
+			if err := updateSnippetMetadata(test.library, tmpDir); err != nil {
+				t.Fatal(err)
+			}
+
+			content, err := os.ReadFile(metadataFile)
+			if err != nil {
+				t.Fatal(err)
+			}
+			s := string(content)
+			if !strings.Contains(s, "$VERSION") {
+				t.Errorf("want version in snippet metadata, got:\n%s", s)
+			}
+		})
+	}
+}
+
 func TestBuildGAPICImportPath(t *testing.T) {
 	for _, test := range []struct {
 		name    string