feat(transport): add user-agent header to client requests. (#457)

* Add a default user-agent header to outgoing requests.
* The user agent can be configured through the `Channel` builder.

fixes #453

tests/integration_tests/tests/user_agent.rs

@@ -0,0 +1,55 @@
+use futures_util::FutureExt;
+use integration_tests::pb::{test_client, test_server, Input, Output};
+use std::time::Duration;
+use tokio::sync::oneshot;
+use tonic::{
+    transport::{Endpoint, Server},
+    Request, Response, Status,
+};
+
+#[tokio::test]
+async fn writes_user_agent_header() {
+    struct Svc;
+
+    #[tonic::async_trait]
+    impl test_server::Test for Svc {
+        async fn unary_call(&self, req: Request<Input>) -> Result<Response<Output>, Status> {
+            match req.metadata().get("user-agent") {
+                Some(_) => Ok(Response::new(Output {})),
+                None => Err(Status::internal("user-agent header is missing")),
+            }
+        }
+    }
+
+    let svc = test_server::TestServer::new(Svc);
+
+    let (tx, rx) = oneshot::channel::<()>();
+
+    let jh = tokio::spawn(async move {
+        Server::builder()
+            .add_service(svc)
+            .serve_with_shutdown("127.0.0.1:1322".parse().unwrap(), rx.map(drop))
+            .await
+            .unwrap();
+    });
+
+    tokio::time::delay_for(Duration::from_millis(100)).await;
+
+    let channel = Endpoint::from_static("http://127.0.0.1:1322")
+        .user_agent("my-client")
+        .expect("valid user agent")
+        .connect()
+        .await
+        .unwrap();
+
+    let mut client = test_client::TestClient::new(channel);
+
+    match client.unary_call(Input {}).await {
+        Ok(_) => {}
+        Err(status) => panic!("{}", status.message()),
+    }
+
+    tx.send(()).unwrap();
+
+    jh.await.unwrap();
+}