implemented server cusotm metadata

tonic-interop/src/lib.rs

@@ -9,7 +9,13 @@ pub mod pb {
     include!(concat!(env!("OUT_DIR"), "/grpc.testing.rs"));
 }
 
-use std::{default, fmt, iter};
+use http::header::{HeaderMap, HeaderName, HeaderValue};
+use http_body::Body;
+use std::{
+    default, fmt, iter,
+    pin::Pin,
+    task::{Context, Poll},
+};
 
 pub fn trace_init() {
     let sub = tracing_subscriber::FmtSubscriber::builder()
@@ -141,3 +147,41 @@ macro_rules! test_assert {
         }
     };
 }
+
+pub struct MergeTrailers<B> {
+    inner: B,
+    trailer: Option<(HeaderName, HeaderValue)>,
+}
+
+impl<B> MergeTrailers<B> {
+    pub fn new(inner: B, trailer: Option<(HeaderName, HeaderValue)>) -> Self {
+        Self { inner, trailer }
+    }
+}
+
+impl<B: Body + Unpin> Body for MergeTrailers<B> {
+    type Data = B::Data;
+    type Error = B::Error;
+
+    fn poll_data(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<Self::Data, Self::Error>>> {
+        Pin::new(&mut self.inner).poll_data(cx)
+    }
+
+    fn poll_trailers(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<Option<HeaderMap>, Self::Error>> {
+        Pin::new(&mut self.inner).poll_trailers(cx).map_ok(|h| {
+            h.map(|mut headers| {
+                if let Some((key, value)) = &self.trailer {
+                    headers.insert(key.clone(), value.clone());
+                }
+
+                headers
+            })
+        })
+    }
+}