feat: Add gRPC interceptors (#232)

This change introduces proper gRPC interceptors that are avilable
regardless of the transport used. Each codegen service now produces an
additional method called `with_interceptor` that accepts a
`Interceptor`.

All examples have been updated to use this new style and interop has a
custom `tower::Service` middleware to echo the headers. There is also a
new `interceptor` example that shows basic usage.

BREAKING CHANGE: removed `interceptor_fn` and `intercep_headers_fn` from `transport` in favor of using `tonic::Interceptor`.

interop/src/bin/server.rs

@@ -1,10 +1,7 @@
-use http::header::HeaderName;
 use structopt::StructOpt;
-use tonic::body::BoxBody;
-use tonic::client::GrpcService;
 use tonic::transport::Server;
 use tonic::transport::{Identity, ServerTlsConfig};
-use tonic_interop::{server, MergeTrailers};
+use tonic_interop::server;
 
 #[derive(StructOpt)]
 struct Opts {
@@ -20,33 +17,7 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
 
     let addr = "127.0.0.1:10000".parse().unwrap();
 
-    let mut builder = Server::builder().interceptor_fn(|svc, req| {
-        let echo_header = req
-            .headers()
-            .get("x-grpc-test-echo-initial")
-            .map(Clone::clone);
-
-        let echo_trailer = req
-            .headers()
-            .get("x-grpc-test-echo-trailing-bin")
-            .map(Clone::clone)
-            .map(|v| (HeaderName::from_static("x-grpc-test-echo-trailing-bin"), v));
-
-        let call = svc.call(req);
-
-        async move {
-            let mut res = call.await?;
-
-            if let Some(echo_header) = echo_header {
-                res.headers_mut()
-                    .insert("x-grpc-test-echo-initial", echo_header);
-            }
-
-            Ok(res
-                .map(|b| MergeTrailers::new(b, echo_trailer))
-                .map(BoxBody::new))
-        }
-    });
+    let mut builder = Server::builder();
 
     if matches.use_tls {
         let cert = tokio::fs::read("interop/data/server1.pem").await?;
@@ -60,8 +31,11 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
     let unimplemented_service =
         server::UnimplementedServiceServer::new(server::UnimplementedService::default());
 
+    // Wrap this test_service with a service that will echo headers as trailers.
+    let test_service_svc = server::EchoHeadersSvc::new(test_service);
+
     builder
-        .add_service(test_service)
+        .add_service(test_service_svc)
         .add_service(unimplemented_service)
         .serve(addr)
         .await?;