fix: harden typed metrics review findings

crates/crank-metrics/src/exemplar.rs

@@ -1,6 +1,6 @@
 use std::{
     collections::BTreeMap,
-    sync::{Mutex, OnceLock},
+    sync::{Arc, Mutex, OnceLock},
 };
 
 use crate::{DURATION_BUCKETS_SECONDS, max_exemplar_slots};
@@ -44,8 +44,9 @@ struct ExemplarKey {
     bucket_index: usize,
 }
 
-fn store() -> &'static Mutex<BTreeMap<ExemplarKey, ExemplarObservation>> {
-    static STORE: OnceLock<Mutex<BTreeMap<ExemplarKey, ExemplarObservation>>> = OnceLock::new();
+fn store() -> &'static Mutex<BTreeMap<ExemplarKey, Arc<ExemplarObservation>>> {
+    static STORE: OnceLock<Mutex<BTreeMap<ExemplarKey, Arc<ExemplarObservation>>>> =
+        OnceLock::new();
     STORE.get_or_init(|| Mutex::new(BTreeMap::new()))
 }
 
@@ -76,20 +77,13 @@ pub(crate) fn record_exemplar(
         bucket_index,
     };
     if observations.contains_key(&key) || observations.len() < max_exemplar_slots() {
-        observations.insert(key, observation);
+        observations.insert(key, Arc::new(observation));
     }
 }
 
-pub fn exemplar_snapshot() -> Vec<ExemplarObservation> {
+pub fn exemplar_snapshot() -> Vec<Arc<ExemplarObservation>> {
     store()
         .lock()
         .map(|observations| observations.values().cloned().collect())
         .unwrap_or_default()
 }
-
-#[doc(hidden)]
-pub fn reset_exemplars_for_test() {
-    if let Ok(mut observations) = store().lock() {
-        observations.clear();
-    }
-}