metrics: rename heartbeat_token_count into heartbeat_tokens

Fix warning from `promtool check metrics`:
  heartbeat_token_count non-histogram and non-summary metrics should not have "_count" suffix

src/metrics.rs

@@ -29,7 +29,7 @@ pub struct Metrics {
     pub heartbeat_registrations_total: Counter,
 
     /// Number of tokens registered for heartbeat notifications.
-    pub heartbeat_token_count: Gauge<i64, AtomicI64>,
+    pub heartbeat_tokens: Gauge<i64, AtomicI64>,
 }
 
 impl Metrics {
@@ -57,19 +57,19 @@ impl Metrics {
             heartbeat_registrations_total.clone(),
         );
 
-        let heartbeat_token_count = Gauge::<i64, AtomicI64>::default();
+        let heartbeat_tokens = Gauge::<i64, AtomicI64>::default();
         registry.register(
-            "heartbeat_token_count",
+            "heartbeat_tokens",
             "Number of tokens registered for heartbeat notifications",
-            heartbeat_token_count.clone(),
+            heartbeat_tokens.clone(),
         );
 
         Self {
             registry,
             direct_notifications_total,
             heartbeat_notifications_total,
             heartbeat_registrations_total,
-            heartbeat_token_count,
+            heartbeat_tokens,
         }
     }
 }

src/notifier.rs

@@ -24,9 +24,7 @@ pub async fn start(state: State, interval: std::time::Duration) -> Result<()> {
     );
 
     loop {
-        metrics
-            .heartbeat_token_count
-            .set(schedule.token_count() as i64);
+        metrics.heartbeat_tokens.set(schedule.token_count() as i64);
 
         let Some((timestamp, token)) = schedule.pop()? else {
             info!("No tokens to notify, sleeping for a minute.");