fix(observability): preserve inbound trace context

apps/admin-api/tests/request_context_parent.rs

@@ -0,0 +1,39 @@
+use admin_api::request_context::{TRACE_ID_HEADER, apply_request_context};
+use axum::{Router, body::Body, http::Request, routing::get};
+use opentelemetry::trace::TracerProvider as _;
+use opentelemetry_sdk::trace::SdkTracerProvider;
+use tower::ServiceExt;
+use tracing_subscriber::layer::SubscriberExt;
+
+#[tokio::test(flavor = "current_thread")]
+async fn preserves_remote_trace_id_with_an_active_tracer_and_no_global_propagator() {
+    let provider = SdkTracerProvider::builder().build();
+    let tracer = provider.tracer("admin-request-context-parent-test");
+    let subscriber =
+        tracing_subscriber::registry().with(tracing_opentelemetry::layer().with_tracer(tracer));
+    let dispatch = tracing::Dispatch::new(subscriber);
+    let _dispatch_guard = tracing::dispatcher::set_default(&dispatch);
+    let app = Router::new()
+        .route("/probe", get(|| async { "ok" }))
+        .layer(axum::middleware::from_fn(apply_request_context));
+
+    let response = app
+        .oneshot(
+            Request::builder()
+                .uri("/probe")
+                .header(
+                    "traceparent",
+                    "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01",
+                )
+                .body(Body::empty())
+                .unwrap(),
+        )
+        .await
+        .unwrap();
+
+    assert_eq!(
+        response.headers()[TRACE_ID_HEADER],
+        "0af7651916cd43dd8448eb211c80319c"
+    );
+    provider.shutdown().unwrap();
+}