[BUG][JAVA] RestTemplate uses hardcoded User-Agent Java-SDK #21972 (#21973)

Co-authored-by: Simon Baranov <simon.baranov@zahlungswerk.de>

modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache

@@ -145,7 +145,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
         this.dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
 
         // Set default User-Agent.
-        setUserAgent("Java-SDK");
+        setUserAgent("{{{httpUserAgent}}}{{^httpUserAgent}}OpenAPI-Generator/{{{artifactVersion}}}/java{{/httpUserAgent}}");
 
         // Setup authentications (key: authentication name, value: authentication).
         authentications = new HashMap<String, Authentication>();{{#authMethods}}{{#isBasic}}{{#isBasicBasic}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -3151,6 +3151,43 @@ public class JavaClientCodegenTest {
 
     }
 
+    @Test
+    public void testRestTemplateWithDefaultUserAgent() {
+
+        final Map<String, File> files = generateFromContract(
+                "src/test/resources/3_1/java/petstore.yaml",
+                JavaClientCodegen.RESTTEMPLATE
+        );
+
+        final JavaFileAssert apiClient = JavaFileAssert.assertThat(files.get("ApiClient.java"))
+                .printFileContent();
+        apiClient
+                .assertMethod("init")
+                .bodyContainsLines("setUserAgent(\"OpenAPI-Generator/1.0.0/java\");");
+    }
+
+    @Test
+    public void testRestTemplateWithCustomUserAgent() {
+
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setValidateSpec(false)
+                .setGeneratorName(JAVA_GENERATOR)
+                .setLibrary(JavaClientCodegen.RESTTEMPLATE)
+                .setHttpUserAgent("MyAwesomeCustomService/0.0.1")
+                .setInputSpec("src/test/resources/3_1/java/petstore.yaml")
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        final Map<String, File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate()
+                .stream().collect(Collectors.toMap(File::getName, Function.identity()));;
+
+        final JavaFileAssert apiClient = JavaFileAssert.assertThat(files.get("ApiClient.java"))
+                .printFileContent();
+        apiClient
+                .assertMethod("init")
+                .bodyContainsLines("setUserAgent(\"MyAwesomeCustomService/0.0.1\");");
+    }
+
     @Test
     public void testRestClientWithGeneratedOAuthTokenSuppliers() {
         final Map<String, File> files = generateFromContract(
@@ -3195,6 +3232,7 @@ public class JavaClientCodegenTest {
         );
     }
 
+
     @Test
     public void testRestClientWithUseSingleRequestParameter_issue_19406() {
         final Path output = newTempFolder();