fix(pageserver): use better estimation for compaction memory usage (#11904)

## Problem

Hopefully resolves `test_gc_feedback` flakiness.

## Summary of changes

`accumulated_values` should not exceed 512MB to avoid OOM. Previously we
only use number of items, which is not a good estimation.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/pageserver_api/src/value.rs

@@ -36,6 +36,24 @@ impl Value {
             Value::WalRecord(rec) => rec.will_init(),
         }
     }
+
+    #[inline(always)]
+    pub fn estimated_size(&self) -> usize {
+        match self {
+            Value::Image(image) => image.len(),
+            Value::WalRecord(NeonWalRecord::AuxFile {
+                content: Some(content),
+                ..
+            }) => content.len(),
+            Value::WalRecord(NeonWalRecord::Postgres { rec, .. }) => rec.len(),
+            Value::WalRecord(NeonWalRecord::ClogSetAborted { xids }) => xids.len() * 4,
+            Value::WalRecord(NeonWalRecord::ClogSetCommitted { xids, .. }) => xids.len() * 4,
+            Value::WalRecord(NeonWalRecord::MultixactMembersCreate { members, .. }) => {
+                members.len() * 8
+            }
+            _ => 8192, /* use image size as the estimation */
+        }
+    }
 }
 
 #[derive(Debug, PartialEq)]