refactor(otel-thread-ctx)!: gate behind feature (#1843)

# What does this PR do?

Moves the `otel-thread-ctx` feature behind a Cargo feature.

# Motivation

This feature broke the dd-trace-php CI because they build with clang 17, and `-mtls-dialect=gnu2` is not recognized there.

I expect this would be problematic for some customers as well. So it seems like a good idea for now to feature flag it.

# Additional Notes

I think the TracerMetadata stuff is okay since it's not `repr(C)` but someone should double-check if this is okay or not, maybe that part needs to always be there and just use empty/default values.

# How to test the change?

Test as usual, and you can test again when enabling feature `otel-thread-ctx`, for example:

```sh
cargo test \
    -p libdd-profiling\
    -p libdd-library-config \
    --features otel-thread-ctx
```

Or:

```sh
cargo nextest run \
    -p libdd-profiling \
    -p libdd-library-config \
    --features otel-thread-ctx 
```



Co-authored-by: yannham <yann.hamdaoui@datadoghq.com>
Co-authored-by: bob.weinand <bob.weinand@datadoghq.com>

Author: 3 people

libdd-library-config/Cargo.toml

@@ -14,6 +14,9 @@ license.workspace = true
 crate-type = ["lib"]
 bench = false
 
+[features]
+otel-thread-ctx = []
+
 [dependencies]
 serde = { version = "1.0", features = ["derive"] }
 serde_yaml = "0.9.34"

libdd-library-config/src/tracer_metadata.rs

@@ -40,6 +40,7 @@ pub struct TracerMetadata {
     /// This field is specific to OTel process context. It is ignored for (de)serialization, and is
     /// only used when converting to an OTel process context in
     /// [TracerMetadata::to_otel_process_ctx].
+    #[cfg(feature = "otel-thread-ctx")]
     #[serde(skip)]
     pub threadlocal_attribute_keys: Vec<String>,
 }
@@ -57,6 +58,7 @@ impl Default for TracerMetadata {
             service_version: None,
             process_tags: None,
             container_id: None,
+            #[cfg(feature = "otel-thread-ctx")]
             threadlocal_attribute_keys: vec![],
         }
     }
@@ -67,8 +69,10 @@ impl TracerMetadata {
     const OTEL_SDK_NAME: &str = "libdatadog";
 
     pub fn to_otel_process_ctx(&self) -> otel_proto::common::v1::ProcessContext {
+        #[cfg(feature = "otel-thread-ctx")]
+        use otel_proto::common::v1::ArrayValue;
         use otel_proto::{
-            common::v1::{any_value, AnyValue, ArrayValue, KeyValue, ProcessContext},
+            common::v1::{any_value, AnyValue, KeyValue, ProcessContext},
             resource::v1::Resource,
         };
 
@@ -102,9 +106,11 @@ impl TracerMetadata {
             service_version,
             process_tags,
             container_id,
+            #[cfg(feature = "otel-thread-ctx")]
             threadlocal_attribute_keys,
         } = self;
 
+        #[cfg_attr(not(feature = "otel-thread-ctx"), allow(unused_mut))]
         let mut attributes = vec![
             key_value_opt("service.name", service_name),
             key_value_opt("service.instance.id", runtime_id),
@@ -117,6 +123,7 @@ impl TracerMetadata {
             key_value_opt("container.id", container_id),
         ];
 
+        #[cfg(feature = "otel-thread-ctx")]
         if !threadlocal_attribute_keys.is_empty() {
             attributes.push(key_value(
                 "threadlocal.schema_version",
@@ -217,9 +224,9 @@ pub use other::*;
 #[cfg(test)]
 mod tests {
     use super::*;
-    use libdd_trace_protobuf::opentelemetry::proto::common::v1::{
-        any_value, AnyValue, ProcessContext,
-    };
+    #[cfg(feature = "otel-thread-ctx")]
+    use libdd_trace_protobuf::opentelemetry::proto::common::v1::any_value;
+    use libdd_trace_protobuf::opentelemetry::proto::common::v1::{AnyValue, ProcessContext};
 
     fn find_attr<'a>(ctx: &'a ProcessContext, key: &str) -> Option<&'a AnyValue> {
         ctx.resource
@@ -239,6 +246,7 @@ mod tests {
         assert!(find_attr(&ctx, "threadlocal.attribute_key_map").is_none());
     }
 
+    #[cfg(feature = "otel-thread-ctx")]
     #[test]
     fn threadlocal_attrs_present_with_correct_values() {
         let ctx = TracerMetadata {

libdd-profiling/Cargo.toml

@@ -19,6 +19,7 @@ bench = false
 [features]
 default = []
 cxx = ["dep:cxx", "dep:cxx-build"]
+otel-thread-ctx = []
 test-utils = ["libdd-common/test-utils"]
 
 [[bench]]

libdd-profiling/build.rs

@@ -12,8 +12,8 @@ fn main() {
         println!("cargo:rerun-if-changed=src/cxx.rs");
     }
 
-    // Only compile the TLS shim on Linux; the thread-level context feature is Linux-only.
-    #[cfg(target_os = "linux")]
+    // Only compile the TLS shim on Linux with the otel-thread-ctx feature enabled.
+    #[cfg(all(target_os = "linux", feature = "otel-thread-ctx"))]
     {
         let mut build = cc::Build::new();
 

libdd-profiling/src/lib.rs

@@ -14,6 +14,7 @@ pub mod cxx;
 pub mod exporter;
 pub mod internal;
 pub mod iter;
+#[cfg(feature = "otel-thread-ctx")]
 pub mod otel_thread_ctx;
 pub mod pprof;
 pub mod profiles;