pageserver: add max_logical_size_per_shard for get_top_tenants (#11157)

## Problem

In #11122, we want to split shards once the logical size of the largest
timeline exceeds a split threshold. However, `get_top_tenants` currently
only returns `max_logical_size`, which tracks the max _total_ logical
size of a timeline across all shards.

This is problematic, because the storage controller needs to fetch a
list of N tenants that are eligible for splits, but the API doesn't
currently have a way to express this. For example, with a split
threshold of 1 GB, a tenant with `max_logical_size` of 4 GB is eligible
to split if it has 1 or 2 shards, but not if it already has 4 shards. We
need to express this in per-shard terms, otherwise the `get_top_tenants`
endpoint may end up only returning tenants that can't be split, blocking
splits entirely.

Touches https://github.com/neondatabase/neon/pull/11122.
Touches https://github.com/neondatabase/cloud/issues/22532.

## Summary of changes

Add `TenantShardItem::max_logical_size_per_shard` containing
`max_logical_size / shard_count`, and
`TenantSorting::MaxLogicalSizePerShard` to order and filter by it.

libs/pageserver_api/src/models.rs

@@ -1476,8 +1476,14 @@ pub struct TenantScanRemoteStorageResponse {
 #[derive(Serialize, Deserialize, Debug, Clone)]
 #[serde(rename_all = "snake_case")]
 pub enum TenantSorting {
+    /// Total size of layers on local disk for all timelines in a shard.
     ResidentSize,
+    /// The logical size of the largest timeline within a _tenant_ (not shard). Only tracked on
+    /// shard 0, contains the sum across all shards.
     MaxLogicalSize,
+    /// The logical size of the largest timeline within a _tenant_ (not shard), divided by number of
+    /// shards. Only tracked on shard 0, and estimates the per-shard logical size.
+    MaxLogicalSizePerShard,
 }
 
 impl Default for TenantSorting {
@@ -1507,14 +1513,20 @@ pub struct TopTenantShardsRequest {
 pub struct TopTenantShardItem {
     pub id: TenantShardId,
 
-    /// Total size of layers on local disk for all timelines in this tenant
+    /// Total size of layers on local disk for all timelines in this shard.
     pub resident_size: u64,
 
-    /// Total size of layers in remote storage for all timelines in this tenant
+    /// Total size of layers in remote storage for all timelines in this shard.
     pub physical_size: u64,
 
-    /// The largest logical size of a timeline within this tenant
+    /// The largest logical size of a timeline within this _tenant_ (not shard). This is only
+    /// tracked on shard 0, and contains the sum of the logical size across all shards.
     pub max_logical_size: u64,
+
+    /// The largest logical size of a timeline within this _tenant_ (not shard) divided by number of
+    /// shards. This is only tracked on shard 0, and is only an estimate as we divide it evenly by
+    /// shard count, rounded up.
+    pub max_logical_size_per_shard: u64,
 }
 
 #[derive(Serialize, Deserialize, Debug, Default)]

pageserver/src/http/routes.rs

@@ -3223,6 +3223,7 @@ async fn post_top_tenants(
         match order_by {
             TenantSorting::ResidentSize => sizes.resident_size,
             TenantSorting::MaxLogicalSize => sizes.max_logical_size,
+            TenantSorting::MaxLogicalSizePerShard => sizes.max_logical_size_per_shard,
         }
     }
 

pageserver/src/tenant.rs

@@ -3842,6 +3842,7 @@ impl Tenant {
             resident_size: 0,
             physical_size: 0,
             max_logical_size: 0,
+            max_logical_size_per_shard: 0,
         };
 
         for timeline in self.timelines.lock().unwrap().values() {
@@ -3858,6 +3859,10 @@ impl Tenant {
             );
         }
 
+        result.max_logical_size_per_shard = result
+            .max_logical_size
+            .div_ceil(self.tenant_shard_id.shard_count.count() as u64);
+
         result
     }
 }