Add basic interceptor support

tonic-interop/src/bin/server.rs

@@ -1,6 +1,9 @@
 use structopt::StructOpt;
 use tonic::Server;
 use tonic_interop::server;
+// TODO: move GrpcService out of client since it can be used for the
+// server too.
+use tonic::client::GrpcService;
 
 #[derive(StructOpt)]
 struct Opts {
@@ -26,6 +29,16 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
         builder.tls(ca, key);
     }
 
+    builder.interceptor_fn(|svc, req| {
+        println!("INBOUND REQUEST={:?}", req);
+        let call = svc.call(req);
+        async move {
+            let res = call.await?;
+            println!("OUTBOUND RESPONSE={:?}", res);
+            Ok(res)
+        }
+    });
+
     builder.serve(addr, test_service).await?;
 
     Ok(())