Use serde_bytes for meta_struct

Otherwise meta_struct will be serialized as an array of integers instead of a byte string, breaking the expected representation.

Signed-off-by: Bob Weinand <bob.weinand@datadoghq.com>

trace-protobuf/build.rs

@@ -67,6 +67,10 @@ fn generate_protobuf() {
         ".pb.Span.error",
         "#[serde(skip_serializing_if = \"is_default\")]",
     );
+    config.field_attribute(
+        ".pb.Span.meta_struct",
+        "#[serde(serialize_with = \"serialize_meta_struct\")]",
+    );
 
     config.type_attribute("StatsPayload", "#[derive(Deserialize, Serialize)]");
     config.type_attribute("StatsPayload", "#[serde(rename_all = \"PascalCase\")]");
@@ -131,6 +135,33 @@ fn generate_protobuf() {
         )
         .unwrap();
 
+    let fn_serialize_meta_struct = "
+fn serialize_meta_struct<S>(input: &::std::collections::HashMap<
+        ::prost::alloc::string::String,
+        ::prost::alloc::vec::Vec<u8>,
+    >, serializer: S) -> Result<S::Ok, S::Error>
+where S: serde::Serializer
+{
+    unsafe {
+        ::std::mem::transmute::<&::std::collections::HashMap<
+            ::prost::alloc::string::String,
+            ::prost::alloc::vec::Vec<u8>,
+        >, &::std::collections::HashMap<
+            ::prost::alloc::string::String,
+            BytesWrapper,
+        >>(input)
+    }.serialize(serializer)
+}
+
+#[derive(Serialize)]
+#[repr(transparent)]
+struct BytesWrapper(#[serde(with = \"serde_bytes\")] Vec<u8>);
+
+"
+    .as_bytes();
+
+    prepend_to_file(fn_serialize_meta_struct, &output_path.join("pb.rs"));
+
     // add license, serde imports, custom deserializer code to the top of the protobuf rust structs
     // file
     let add_to_top = "// Copyright 2023-Present Datadog, Inc. https://www.datadoghq.com/

trace-protobuf/src/pb.rs

@@ -16,6 +16,27 @@ pub fn is_default<T: Default + PartialEq>(t: &T) -> bool {
     t == &T::default()
 }
 
+
+fn serialize_meta_struct<S>(input: &::std::collections::HashMap<
+        ::prost::alloc::string::String,
+        ::prost::alloc::vec::Vec<u8>,
+    >, serializer: S) -> Result<S::Ok, S::Error>
+where S: serde::Serializer
+{
+    unsafe {
+        ::std::mem::transmute::<&::std::collections::HashMap<
+            ::prost::alloc::string::String,
+            ::prost::alloc::vec::Vec<u8>,
+        >, &::std::collections::HashMap<
+            ::prost::alloc::string::String,
+            BytesWrapper,
+        >>(input)
+    }.serialize(serializer)
+}
+
+#[derive(Serialize)]
+struct BytesWrapper(#[serde(with = "serde_bytes")] Vec<u8>);
+
 #[derive(Deserialize, Serialize)]
 #[allow(clippy::derive_partial_eq_without_eq)]
 #[derive(Clone, PartialEq, ::prost::Message)]
@@ -140,6 +161,7 @@ pub struct Span {
     #[serde(default)]
     #[serde(deserialize_with = "deserialize_null_into_default")]
     #[serde(skip_serializing_if = "::std::collections::HashMap::is_empty")]
+    #[serde(serialize_with = "serialize_meta_struct")]
     pub meta_struct: ::std::collections::HashMap<
         ::prost::alloc::string::String,
         ::prost::alloc::vec::Vec<u8>,