fix: do TimelineMetrics::shutdown only once (#7983)

Related to #7341 tenant deletion will end up shutting down timelines
twice, once before actually starting and the second time when per
timeline deletion is requested. Shutting down TimelineMetrics causes
underflows. Add an atomic boolean and only do the shutdown once.

pageserver/src/metrics.rs

@@ -2108,6 +2108,7 @@ pub(crate) struct TimelineMetrics {
     pub directory_entries_count_gauge: Lazy<UIntGauge, Box<dyn Send + Fn() -> UIntGauge>>,
     pub evictions: IntCounter,
     pub evictions_with_low_residence_duration: std::sync::RwLock<EvictionsWithLowResidenceDuration>,
+    shutdown: std::sync::atomic::AtomicBool,
 }
 
 impl TimelineMetrics {
@@ -2227,6 +2228,7 @@ impl TimelineMetrics {
             evictions_with_low_residence_duration: std::sync::RwLock::new(
                 evictions_with_low_residence_duration,
             ),
+            shutdown: std::sync::atomic::AtomicBool::default(),
         }
     }
 
@@ -2249,6 +2251,17 @@ impl TimelineMetrics {
     }
 
     pub(crate) fn shutdown(&self) {
+        let was_shutdown = self
+            .shutdown
+            .swap(true, std::sync::atomic::Ordering::Relaxed);
+
+        if was_shutdown {
+            // this happens on tenant deletion because tenant first shuts down timelines, then
+            // invokes timeline deletion which first shuts down the timeline again.
+            // TODO: this can be removed once https://github.com/neondatabase/neon/issues/5080
+            return;
+        }
+
         let tenant_id = &self.tenant_id;
         let timeline_id = &self.timeline_id;
         let shard_id = &self.shard_id;