fix: harden typed metrics review findings

crates/crank-community-mcp/src/request_context.rs

@@ -1,5 +1,8 @@
+use std::time::Instant;
+
 use axum::{extract::Request, http::HeaderValue, middleware::Next, 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_span};
 
@@ -10,12 +13,25 @@ const HEADER_X_TRACE_ID: axum::http::HeaderName = axum::http::HeaderName::from_s
 #[derive(Clone, Debug)]
 pub(super) struct RequestContext {
     pub(super) correlation: CorrelationContext,
+    started_at: Instant,
 }
 
 impl RequestContext {
     pub(super) fn request_id(&self) -> &str {
         self.correlation.request_id().as_str()
     }
+
+    pub(super) fn started_at(&self) -> Instant {
+        self.started_at
+    }
+
+    pub(super) fn exemplar(&self) -> Option<ExemplarTraceId> {
+        self.correlation
+            .trace_context()
+            .is_sampled()
+            .then(|| ExemplarTraceId::parse(self.correlation.trace_id().as_str()))
+            .flatten()
+    }
 }
 
 pub(super) async fn apply_request_context(mut request: Request, next: Next) -> Response {
@@ -37,6 +53,7 @@ pub(super) async fn apply_request_context(mut request: Request, next: Next) -> R
     span.record("trace_id", trace_context.trace_id().as_str());
     let context = RequestContext {
         correlation: CorrelationContext::new(request_id, trace_context),
+        started_at: Instant::now(),
     };
     request.extensions_mut().insert(context.clone());
 
@@ -51,6 +68,9 @@ pub(super) async fn apply_request_context(mut request: Request, next: Next) -> R
             if let Ok(value) = HeaderValue::from_str(context.correlation.trace_id().as_str()) {
                 response.headers_mut().insert(HEADER_X_TRACE_ID, value);
             }
+            if let Some(exemplar) = context.exemplar() {
+                response.extensions_mut().insert(exemplar);
+            }
             response
         },
     )