feat: freeze typed metrics registry and bounded exemplars

crates/crank-community-mcp/src/app/metrics.rs

@@ -1,9 +1,9 @@
-use std::sync::Arc;
+use std::{sync::Arc, time::Instant};
 
 use axum::{http::header::CONTENT_TYPE, response::Response};
 use crank_metrics::{
-    InFlightGuard, LimitStage, McpMethod, McpOutcome, McpResponseMode, record_limit_rejection,
-    record_mcp_request,
+    ExemplarTraceId, InFlightGuard, LimitStage, McpMethod, McpOutcome, McpResponseMode,
+    record_limit_rejection, record_mcp_request_with_exemplar,
 };
 use serde_json::Value;
 use tokio::sync::{OwnedSemaphorePermit, Semaphore};
@@ -14,14 +14,18 @@ pub(super) struct McpRequestMetrics {
     method: McpMethod,
     response_mode: McpResponseMode,
     outcome: McpOutcome,
+    started_at: Instant,
+    exemplar: Option<ExemplarTraceId>,
 }
 
 impl McpRequestMetrics {
-    pub(super) const fn invalid() -> Self {
+    pub(super) fn invalid() -> Self {
         Self {
             method: McpMethod::Invalid,
             response_mode: McpResponseMode::Unknown,
             outcome: McpOutcome::Aborted,
+            started_at: Instant::now(),
+            exemplar: None,
         }
     }
 
@@ -30,10 +34,17 @@ impl McpRequestMetrics {
             method: normalized_mcp_method(message),
             response_mode: McpResponseMode::Unknown,
             outcome: McpOutcome::Aborted,
+            started_at: Instant::now(),
+            exemplar: None,
         }
     }
 
     pub(super) fn complete(&mut self, response: &Response) {
+        self.exemplar = response
+            .headers()
+            .get("x-trace-id")
+            .and_then(|value| value.to_str().ok())
+            .and_then(ExemplarTraceId::parse);
         self.response_mode = response
             .headers()
             .get(CONTENT_TYPE)
@@ -71,7 +82,13 @@ impl McpRequestMetrics {
 
 impl Drop for McpRequestMetrics {
     fn drop(&mut self) {
-        record_mcp_request(self.method, self.response_mode, self.outcome);
+        record_mcp_request_with_exemplar(
+            self.method,
+            self.response_mode,
+            self.outcome,
+            self.started_at.elapsed(),
+            self.exemplar,
+        );
     }
 }
 

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

@@ -79,8 +79,12 @@ impl ActiveSessionMetrics {
                 )
                 .await
                 {
-                    Ok(Ok(count)) => crank_metrics::set_mcp_active_sessions(count),
+                    Ok(Ok(count)) => {
+                        crank_metrics::set_mcp_active_sessions(count);
+                        crank_metrics::set_mcp_session_metrics_fresh(true);
+                    }
                     Ok(Err(_)) | Err(_) => {
+                        crank_metrics::set_mcp_session_metrics_fresh(false);
                         warn!(
                             name: "mcp.active_session_metrics.refresh_failed",
                             error_category = "session_store",