feat: freeze typed metrics registry and bounded exemplars

crates/crank-observability/tests/metrics_signals.rs

@@ -17,8 +17,7 @@ use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
 #[tokio::test]
 async fn typed_product_signals_are_exposed_by_a_real_scrape() {
-    let identity =
-        ServiceIdentity::try_new("metrics-signals", "0.3.1", "test").expect("valid identity");
+    let identity = ServiceIdentity::try_new("admin-api", "0.3.1", "test").expect("valid identity");
     let lifecycle = crank_observability::init(ObservabilityConfig::new(
         identity,
         "off",
@@ -49,6 +48,7 @@ async fn typed_product_signals_are_exposed_by_a_real_scrape() {
         McpMethod::ToolsCall,
         McpResponseMode::Json,
         McpOutcome::ToolError,
+        Duration::from_millis(1),
     );
     set_mcp_active_sessions(2);
     let _stream = InFlightGuard::mcp_stream();