feat: user-agent over gRPC (#2840)

Author: dbolduc

src/gax-internal/src/grpc.rs

@@ -72,7 +72,7 @@ impl Client {
         Request: prost::Message + 'static + Clone,
         Response: prost::Message + Default + 'static,
     {
-        let headers = Self::make_headers(api_client_header, request_params).await?;
+        let headers = Self::make_headers(api_client_header, request_params, &options).await?;
         match self.get_retry_policy(&options) {
             None => {
                 self.request_attempt::<Request, Response>(
@@ -204,8 +204,15 @@ impl Client {
     async fn make_headers(
         api_client_header: &'static str,
         request_params: &str,
+        options: &gax::options::RequestOptions,
     ) -> Result<http::header::HeaderMap> {
         let mut headers = HeaderMap::new();
+        if let Some(user_agent) = options.user_agent() {
+            headers.append(
+                http::header::USER_AGENT,
+                http::header::HeaderValue::from_str(user_agent).map_err(Error::ser)?,
+            );
+        }
         headers.append(
             http::header::HeaderName::from_static("x-goog-api-client"),
             http::header::HeaderValue::from_static(api_client_header),

src/gax-internal/tests/grpc_user_agent.rs

@@ -0,0 +1,76 @@
+// Copyright 2025 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
+//
+//     https://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.
+
+#[cfg(all(test, feature = "_internal-grpc-client"))]
+mod test {
+    use gax::options::*;
+    use google_cloud_gax_internal::grpc;
+    use grpc_server::{builder, google, start_echo_server};
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_user_agent() -> anyhow::Result<()> {
+        let (endpoint, _server) = start_echo_server().await?;
+        let client = builder(endpoint)
+            .with_credentials(auth::credentials::testing::test_credentials())
+            .build()
+            .await?;
+
+        let options = {
+            let mut o = RequestOptions::default();
+            o.set_user_agent("custom-user-agent/v1.2.3");
+            o
+        };
+        let response = send_request(client, options).await?;
+        let user_agent = response
+            .metadata
+            .get(http::header::USER_AGENT.as_str())
+            .map(String::as_str)
+            .expect("There should be a User-Agent header");
+        let components: Vec<&str> = user_agent.split(' ').collect();
+        assert!(
+            components.contains(&"custom-user-agent/v1.2.3"),
+            "User-Agent: {user_agent}"
+        );
+        Ok(())
+    }
+
+    async fn send_request(
+        client: grpc::Client,
+        options: gax::options::RequestOptions,
+    ) -> gax::Result<google::test::v1::EchoResponse> {
+        let extensions = {
+            let mut e = tonic::Extensions::new();
+            e.insert(tonic::GrpcMethod::new(
+                "google.test.v1.EchoServices",
+                "Echo",
+            ));
+            e
+        };
+        let request = google::test::v1::EchoRequest {
+            message: "message".into(),
+            ..google::test::v1::EchoRequest::default()
+        };
+        client
+            .execute(
+                extensions,
+                http::uri::PathAndQuery::from_static("/google.test.v1.EchoService/Echo"),
+                request,
+                options,
+                "test-only-api-client/1.0",
+                "",
+            )
+            .await
+            .map(tonic::Response::into_inner)
+    }
+}