add ancient pack metrics (solana-labs#2206)

accounts-db/src/accounts_db.rs

@@ -1957,6 +1957,8 @@ pub(crate) struct ShrinkAncientStats {
     pub(crate) many_ref_slots_skipped: AtomicU64,
     pub(crate) slots_cannot_move_count: AtomicU64,
     pub(crate) many_refs_old_alive: AtomicU64,
+    pub(crate) slots_eligible_to_shrink: AtomicU64,
+    pub(crate) total_dead_bytes: AtomicU64,
 }
 
 #[derive(Debug, Default)]
@@ -2239,6 +2241,16 @@ impl ShrinkAncientStats {
                 self.random_shrink.swap(0, Ordering::Relaxed) as i64,
                 i64
             ),
+            (
+                "slots_eligible_to_shrink",
+                self.slots_eligible_to_shrink.swap(0, Ordering::Relaxed),
+                i64
+            ),
+            (
+                "total_dead_bytes",
+                self.total_dead_bytes.swap(0, Ordering::Relaxed),
+                i64
+            ),
             (
                 "slots_considered",
                 self.slots_considered.swap(0, Ordering::Relaxed) as i64,

accounts-db/src/ancient_append_vecs.rs

@@ -572,6 +572,20 @@ impl AccountsDb {
                 }
             }
         }
+        let mut total_dead_bytes = 0;
+        let should_shrink_count = infos
+            .all_infos
+            .iter()
+            .filter(|info| info.should_shrink)
+            .map(|info| total_dead_bytes += info.capacity.saturating_sub(info.alive_bytes))
+            .count()
+            .saturating_sub(randoms as usize);
+        self.shrink_ancient_stats
+            .slots_eligible_to_shrink
+            .fetch_add(should_shrink_count as u64, Ordering::Relaxed);
+        self.shrink_ancient_stats
+            .total_dead_bytes
+            .fetch_add(total_dead_bytes, Ordering::Relaxed);
         if randoms > 0 {
             self.shrink_ancient_stats
                 .random_shrink