impl(o11y): introduce gcp.client.version (#4140)

This PR introduces a new tracing attribute `gcp.client.version` to
include the client library version in observability data.

The changes include:

- **hermetic build**:
- The hermetic build scripts are updated to automatically add an entry
for the new `Version.java` file in the `.OwlBot-hermetic.yaml` file for
newly generated libraries.

- **release-please**:
- `release-please-config.json` is updated to ensure that the
`Version.java` file in the showcase client is updated during the release
process.

- Golden files and other generated clients are updated to reflect these
changes.

### Downstream Impact

This change will require modifications in downstream client libraries to
adopt the new `Version.java` file. Specifically:

- `.OwlBot.yaml` files will need to be updated to preserve the
`Version.java` file during OwlBot runs. The hermetic build scripts will
handle this for new libraries, but existing libraries will need to be
updated.
- `release-please.config.json` files in downstream repositories will
need to be updated to include the `Version.java` file in the
`extra-files` section to enable version updates by `release-please`.

### Manual testing

Release please does modify the version string. See
https://github.com/googleapis/sdk-platform-java/pull/4141/changes#diff-7cc6cb6679ef4c6139adc5d9cdc13084773ef22e919bf2e2dce21120c76d6787

Author: diegomarquezp

gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ClassDefinition.java

@@ -203,9 +203,6 @@ public ClassDefinition build() {
                 expr instanceof AssignmentExpr,
                 "Class expression statement must be assignment or variable declaration");
             VariableExpr variableExpr = ((AssignmentExpr) expr).variableExpr();
-            Preconditions.checkState(
-                !variableExpr.scope().equals(ScopeNode.LOCAL),
-                "Class variable in assignment statement cannot have a local scope");
           }
         }
       }

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java

@@ -69,6 +69,7 @@ public static List<GapicClass> generateServiceClasses(GapicContext context) {
     clazzes.addAll(generateClientSettingsClasses(context));
     clazzes.addAll(generateMockClasses(context, context.services()));
     clazzes.addAll(generateTestClasses(context));
+    clazzes.addAll(generateVersionClasses(context));
     return clazzes;
   }
 
@@ -197,6 +198,15 @@ public static List<GapicClass> generateTestClasses(GapicContext context) {
     return clazzes;
   }
 
+  public static List<GapicClass> generateVersionClasses(GapicContext context) {
+    return context.services().stream()
+        .collect(Collectors.toMap(Service::pakkage, s -> s, (s1, s2) -> s1))
+        .values()
+        .stream()
+        .map(service -> LibraryVersionClassComposer.instance().generate(context, service))
+        .collect(Collectors.toList());
+  }
+
   @VisibleForTesting
   static List<GapicClass> prepareExecutableSamples(List<GapicClass> clazzes) {
     // Include license header, apiShortName, and apiVersion

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/LibraryVersionClassComposer.java

@@ -0,0 +1,95 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.api.generator.gapic.composer;
+
+import com.google.api.generator.engine.ast.AnnotationNode;
+import com.google.api.generator.engine.ast.AssignmentExpr;
+import com.google.api.generator.engine.ast.ClassDefinition;
+import com.google.api.generator.engine.ast.CommentStatement;
+import com.google.api.generator.engine.ast.ConcreteReference;
+import com.google.api.generator.engine.ast.ExprStatement;
+import com.google.api.generator.engine.ast.LineComment;
+import com.google.api.generator.engine.ast.ScopeNode;
+import com.google.api.generator.engine.ast.StringObjectValue;
+import com.google.api.generator.engine.ast.TypeNode;
+import com.google.api.generator.engine.ast.ValueExpr;
+import com.google.api.generator.engine.ast.Variable;
+import com.google.api.generator.engine.ast.VariableExpr;
+import com.google.api.generator.gapic.model.GapicClass;
+import com.google.api.generator.gapic.model.GapicContext;
+import com.google.api.generator.gapic.model.Service;
+import java.util.Arrays;
+
+public class LibraryVersionClassComposer {
+  private static final LibraryVersionClassComposer INSTANCE = new LibraryVersionClassComposer();
+
+  private LibraryVersionClassComposer() {}
+
+  public static LibraryVersionClassComposer instance() {
+    return INSTANCE;
+  }
+
+  public GapicClass generate(GapicContext context, Service service) {
+    String packageName = service.pakkage() + ".stub";
+    String className = "Version";
+
+    String artifact = context.artifact();
+    String artifactId = artifact;
+    if (artifact != null && artifact.contains(":")) {
+      artifactId = artifact.split(":")[1];
+    }
+
+    VariableExpr versionVarExpr =
+        VariableExpr.withVariable(
+            Variable.builder().setType(TypeNode.STRING).setName("VERSION").build());
+    AssignmentExpr versionAssignmentExpr =
+        AssignmentExpr.builder()
+            .setVariableExpr(
+                versionVarExpr.toBuilder()
+                    .setIsDecl(true)
+                    .setScope(ScopeNode.LOCAL)
+                    .setIsStatic(true)
+                    .setIsFinal(true)
+                    .build())
+            .setValueExpr(ValueExpr.withValue(StringObjectValue.withValue("0.0.0-SNAPSHOT")))
+            .build();
+
+    ClassDefinition classDef =
+        ClassDefinition.builder()
+            .setPackageString(packageName)
+            .setAnnotations(
+                Arrays.asList(
+                    AnnotationNode.builder()
+                        .setType(
+                            TypeNode.withReference(
+                                ConcreteReference.withClazz(com.google.api.core.InternalApi.class)))
+                        .setDescription("For internal use only")
+                        .build()))
+            .setScope(ScopeNode.LOCAL)
+            .setIsFinal(true)
+            .setName(className)
+            .setStatements(
+                Arrays.asList(
+                    CommentStatement.withComment(
+                        LineComment.withComment(
+                            String.format("{x-version-update-start:%s:current}", artifactId))),
+                    ExprStatement.withExpr(versionAssignmentExpr),
+                    CommentStatement.withComment(
+                        LineComment.withComment("{x-version-update-end}"))))
+            .build();
+
+    return GapicClass.create(GapicClass.Kind.MAIN, classDef);
+  }
+}

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java

@@ -1063,7 +1063,7 @@ private List<MethodDefinition> createClassMethods(
             SettingsCommentComposer.NEW_BUILDER_METHOD_COMMENT));
     javaMethods.addAll(createBuilderHelperMethods(service, typeStore));
     javaMethods.add(createClassConstructor(service, methodSettingsMemberVarExprs, typeStore));
-    javaMethods.add(createGetLibraryMetadataMethod(context));
+    javaMethods.add(createGetLibraryMetadataMethod(context, service));
     return javaMethods;
   }
 
@@ -2107,7 +2107,7 @@ private static MethodDefinition createNestedClassBuildMethod(
         .build();
   }
 
-  private MethodDefinition createGetLibraryMetadataMethod(GapicContext context) {
+  private MethodDefinition createGetLibraryMetadataMethod(GapicContext context, Service service) {
     TypeNode returnType = FIXED_TYPESTORE.get("LibraryMetadata");
     MethodInvocationExpr libraryMetadataBuilderExpr =
         MethodInvocationExpr.builder()
@@ -2133,6 +2133,23 @@ private MethodDefinition createGetLibraryMetadataMethod(GapicContext context) {
               .build();
     }
 
+    libraryMetadataBuilderExpr =
+        MethodInvocationExpr.builder()
+            .setExprReferenceExpr(libraryMetadataBuilderExpr)
+            .setMethodName("setVersion")
+            .setArguments(
+                VariableExpr.builder()
+                    .setStaticReferenceType(
+                        TypeNode.withReference(
+                            VaporReference.builder()
+                                .setName("Version")
+                                .setPakkage(String.format("%s.stub", service.pakkage()))
+                                .build()))
+                    .setVariable(
+                        Variable.builder().setName("VERSION").setType(TypeNode.STRING).build())
+                    .build())
+            .build();
+
     Expr returnExpr =
         MethodInvocationExpr.builder()
             .setExprReferenceExpr(libraryMetadataBuilderExpr)

gapic-generator-java/src/test/java/com/google/api/generator/engine/ast/ClassDefinitionTest.java

@@ -293,31 +293,6 @@ void invalidClassDefinition_extendsImplementsSameType() {
         });
   }
 
-  @Test
-  void invalidClassDefinition_assignmentWithUnscopedVariableExprStatement() {
-    Variable variable = createVariable("x", TypeNode.INT);
-    VariableExpr variableExpr =
-        VariableExpr.builder().setVariable(variable).setIsDecl(true).build();
-
-    Variable anotherVariable = createVariable("y", TypeNode.INT);
-    Expr valueExpr = VariableExpr.builder().setVariable(anotherVariable).build();
-
-    AssignmentExpr assignmentExpr =
-        AssignmentExpr.builder().setVariableExpr(variableExpr).setValueExpr(valueExpr).build();
-
-    List<Statement> statements = Arrays.asList(ExprStatement.withExpr(assignmentExpr));
-    assertThrows(
-        IllegalStateException.class,
-        () -> {
-          ClassDefinition.builder()
-              .setPackageString("com.google.example.library.v1.stub")
-              .setName("LibraryServiceStub")
-              .setScope(ScopeNode.PUBLIC)
-              .setStatements(statements)
-              .build();
-        });
-  }
-
   @Test
   void invalidClassDefinition_unscopedVariableExprStatement() {
     List<Statement> statements =

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/LibraryVersionClassComposerTest.java

@@ -0,0 +1,34 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.api.generator.gapic.composer;
+
+import com.google.api.generator.gapic.model.GapicClass;
+import com.google.api.generator.gapic.model.GapicContext;
+import com.google.api.generator.gapic.model.Service;
+import com.google.api.generator.test.framework.Assert;
+import com.google.api.generator.test.protoloader.GrpcTestProtoLoader;
+import org.junit.Test;
+
+public class LibraryVersionClassComposerTest {
+
+  @Test
+  public void generateVersionClasses() {
+    GapicContext context = GrpcTestProtoLoader.instance().parseShowcaseEcho();
+    Service service = context.services().get(0);
+    GapicClass clazz = LibraryVersionClassComposer.instance().generate(context, service);
+
+    Assert.assertGoldenClass(this.getClass(), clazz, "EchoVersion.golden");
+  }
+}

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/goldens/EchoVersion.golden

@@ -0,0 +1,11 @@
+package com.google.showcase.v1beta1.stub;
+
+import com.google.api.core.InternalApi;
+
+@InternalApi("For internal use only")
+final class Version {
+  // {x-version-update-start:gapic-showcase:current}
+  static final String VERSION = "0.0.0-SNAPSHOT";
+  // {x-version-update-end}
+
+}

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/grpc/goldens/ApiVersionTestingStubSettings.golden

@@ -170,7 +170,7 @@ public class EchoWithVersionStubSettings extends StubSettings<EchoWithVersionStu
 
   @Override
   protected LibraryMetadata getLibraryMetadata() {
-    return LibraryMetadata.newBuilder().build();
+    return LibraryMetadata.newBuilder().setVersion(Version.VERSION).build();
   }
 
   /** Builder for EchoWithVersionStubSettings. */

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/grpc/goldens/DeprecatedServiceStubSettings.golden

@@ -187,7 +187,7 @@ public class DeprecatedServiceStubSettings extends StubSettings<DeprecatedServic
 
   @Override
   protected LibraryMetadata getLibraryMetadata() {
-    return LibraryMetadata.newBuilder().build();
+    return LibraryMetadata.newBuilder().setVersion(Version.VERSION).build();
   }
 
   /** Builder for DeprecatedServiceStubSettings. */

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/grpc/goldens/EchoServiceSelectiveGapicStubSettings.golden

@@ -224,7 +224,7 @@ public class EchoServiceShouldGeneratePartialUsualStubSettings
 
   @Override
   protected LibraryMetadata getLibraryMetadata() {
-    return LibraryMetadata.newBuilder().build();
+    return LibraryMetadata.newBuilder().setVersion(Version.VERSION).build();
   }
 
   /** Builder for EchoServiceShouldGeneratePartialUsualStubSettings. */