Merge pull request #34562: [extensions-proto] Return correct TypeDescriptor for ProtoCoder (#34561)

Author: je-ik

sdks/java/extensions/protobuf/src/main/java/org/apache/beam/sdk/extensions/protobuf/ProtoCoder.java

@@ -237,6 +237,11 @@ public Class<T> getMessageType() {
     return protoMessageClass;
   }
 
+  @Override
+  public TypeDescriptor<T> getEncodedTypeDescriptor() {
+    return TypeDescriptor.of(protoMessageClass);
+  }
+
   public Set<Class<?>> getExtensionHosts() {
     return extensionHostClasses;
   }

sdks/java/extensions/protobuf/src/test/java/org/apache/beam/sdk/extensions/protobuf/ProtoCoderTest.java

@@ -169,6 +169,12 @@ public void testNonDeterministicProperty() throws CoderException {
     assertNotEquals(CoderUtils.encodeToBase64(coder, msg2), CoderUtils.encodeToBase64(coder, msg1));
   }
 
+  @Test
+  public void testEncodedTypeDescriptor() {
+    Coder<MessageA> coder = ProtoCoder.of(MessageA.class);
+    assertEquals(TypeDescriptor.of(MessageA.class), coder.getEncodedTypeDescriptor());
+  }
+
   @Test
   public void testSerialVersionID() {
     long serialVersionID = ObjectStreamClass.lookup(ProtoCoder.class).getSerialVersionUID();