Serialize payload in "handle message" span

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/rpc/build.rs

@@ -2,6 +2,7 @@ fn main() {
     let mut build = prost_build::Config::new();
     // build.protoc_arg("--experimental_allow_proto3_optional");
     build
+        .type_attribute(".", "#[derive(serde::Serialize)]")
         .compile_protos(&["proto/zed.proto"], &["proto"])
         .unwrap();
 }