diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index a56d3ff66..dbb62876b 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -1964,6 +1964,7 @@ dependencies = [
  "secrecy",
  "serde",
  "sha2",
+ "smallvec 1.13.2",
  "socket-factory",
  "socket2",
  "stun_codec",
diff --git a/rust/relay/Cargo.toml b/rust/relay/Cargo.toml
index 6be4e0c27..479b19ab7 100644
--- a/rust/relay/Cargo.toml
+++ b/rust/relay/Cargo.toml
@@ -26,6 +26,7 @@ rand = "0.8.5"
 secrecy = { workspace = true }
 serde = { version = "1.0.204", features = ["derive"] }
 sha2 = "0.10.8"
+smallvec = "1.13.2"
 socket-factory = { workspace = true }
 socket2 = { workspace = true }
 stun_codec = "0.3.4"
diff --git a/rust/relay/src/server.rs b/rust/relay/src/server.rs
index da5a6f40c..fba5389be 100644
--- a/rust/relay/src/server.rs
+++ b/rust/relay/src/server.rs
@@ -17,6 +17,7 @@ use opentelemetry::metrics::{Counter, UpDownCounter};
 use opentelemetry::KeyValue;
 use rand::Rng;
 use secrecy::SecretString;
+use smallvec::SmallVec;
 use std::collections::{HashMap, VecDeque};
 use std::hash::Hash;
 use std::net::{IpAddr, SocketAddr};
@@ -862,6 +863,7 @@ where
     fn send_message(&mut self, message: Message<Attribute>, recipient: ClientSocket) {
         let method = message.method();
         let class = message.class();
+        let error_code = message.get_attribute::<ErrorCode>().map(|e| e.code());
         tracing::trace!(target: "relay",  method = %message.method(), class = %message.class(), "Sending message");
 
         let Ok(bytes) = self.encoder.encode_into_bytes(message) else {
@@ -890,13 +892,18 @@ where
             CREATE_PERMISSION => "createpermission",
             _ => return,
         };
-        self.responses_counter.add(
-            1,
-            &[
-                KeyValue::new("response_class", response_class),
-                KeyValue::new("message_type", message_type),
-            ],
-        );
+        let error_code = error_code.map(|c| opentelemetry::Value::from(c as i64));
+
+        // Use a `SmallVec` to avoid heap-allocations when collecting metrics.
+        let mut attributes = SmallVec::<[KeyValue; 3]>::with_capacity(3);
+        attributes.push(KeyValue::new("response_class", response_class));
+        attributes.push(KeyValue::new("message_type", message_type));
+
+        if let Some(error_code) = error_code {
+            attributes.push(KeyValue::new("error_code", error_code));
+        }
+
+        self.responses_counter.add(1, &attributes);
     }
 
     fn delete_allocation(&mut self, port: AllocationPort) {