fix: harden typed metrics review findings

apps/admin-api/src/request_context.rs

@@ -5,6 +5,7 @@ use axum::{
     response::Response,
 };
 use crank_core::{CorrelationContext, RequestId, TraceContext};
+use crank_metrics::ExemplarTraceId;
 use crank_observability::{set_remote_trace_parent, with_request_correlation};
 use tracing::{Instrument, info, info_span};
 
@@ -74,6 +75,12 @@ pub async fn apply_request_context(mut request: Request, next: Next) -> Response
             if let Ok(value) = HeaderValue::from_str(context.correlation.trace_id().as_str()) {
                 response.headers_mut().insert(TRACE_ID_HEADER, value);
             }
+            if context.correlation.trace_context().is_sampled()
+                && let Some(exemplar) =
+                    ExemplarTraceId::parse(context.correlation.trace_id().as_str())
+            {
+                response.extensions_mut().insert(exemplar);
+            }
             response
         },
     )