chore: Append language to user-agent header

FUTURE_COPYBARA_INTEGRATE_REVIEW=#596 from googleapis:release-please--branches--main--components--vertexai 489440d
PiperOrigin-RevId: 900425828

Author: sararob

src/genai/client.ts

@@ -35,7 +35,8 @@ export class Client {
       vertexai: true,
       httpOptions: options.apiEndpoint ? {baseUrl: options.apiEndpoint} :
                                          undefined,
-      userAgentExtra: `vertex-genai-modules/${SDK_VERSION}`,
+      userAgentExtra:
+          `vertex-genai-modules/${SDK_VERSION} gl-node/${process.version}`,
     });
 
     this._agentEnginesInternal = new AgentEngines(this.apiClient);