fix: harden typed metrics review findings

apps/admin-api/Cargo.toml

@@ -24,6 +24,7 @@ crank-config = { path = "../../crates/crank-config" }
 crank-core = { path = "../../crates/crank-core" }
 crank-import = { path = "../../crates/crank-import" }
 crank-mapping = { path = "../../crates/crank-mapping" }
+crank-metrics = { path = "../../crates/crank-metrics" }
 crank-observability = { path = "../../crates/crank-observability" }
 crank-registry = { path = "../../crates/crank-registry" }
 crank-runtime = { path = "../../crates/crank-runtime" }

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
         },
     )