refactor: change Protocol's Serialize and Deserialize to use standard values

BREAKING CHANGE: users depending on the previous values (or having saved them) may observe the change

Author: davidhewitt

opentelemetry-otlp/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## vNext
 
+- **Breaking** `opentelemetry_otlp::Protocol` implementations of `Serialize` and `Deserialize` have been changed to [match standard otel values for protocol](https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/#otel_exporter_otlp_protocol). [#2765](https://github.com/open-telemetry/opentelemetry-rust/pull/2765)
+
 ## 0.30.0
 
 Released 2025-May-23

opentelemetry-otlp/src/lib.rs

@@ -442,10 +442,13 @@ use serde::{Deserialize, Serialize};
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum Protocol {
     /// GRPC protocol
+    #[cfg_attr(feature = "serialize", serde(rename = "grpc"))]
     Grpc,
     /// HTTP protocol with binary protobuf
+    #[cfg_attr(feature = "serialize", serde(rename = "http/protobuf"))]
     HttpBinary,
     /// HTTP protocol with JSON payload
+    #[cfg_attr(feature = "serialize", serde(rename = "http/json"))]
     HttpJson,
 }
 
@@ -470,3 +473,25 @@ pub mod tonic_types {
         pub use tonic::transport::{Certificate, ClientTlsConfig, Identity};
     }
 }
+
+#[cfg(test)]
+mod tests {
+
+    #[cfg(feature = "serialize")]
+    #[test]
+    fn test_protocol_serialization() {
+        use super::Protocol;
+
+        for (protocol, expected) in [
+            (Protocol::Grpc, r#""grpc""#),
+            (Protocol::HttpBinary, r#""http/protobuf""#),
+            (Protocol::HttpJson, r#""http/json""#),
+        ] {
+            let serialized = serde_json::to_string(&protocol).unwrap();
+            assert_eq!(serialized, expected);
+
+            let deserialized: Protocol = serde_json::from_str(&serialized).unwrap();
+            assert_eq!(deserialized, protocol);
+        }
+    }
+}