controller: use PageserverUtilization for scheduling (#8711)

## Problem

Previously, the controller only used the shard counts for scheduling.
This works well when hosting only many-sharded tenants, but works much
less well when hosting single-sharded tenants that have a greater
deviation in size-per-shard.

Closes: https://github.com/neondatabase/neon/issues/7798

## Summary of changes

- Instead of UtilizationScore, carry the full PageserverUtilization
through into the Scheduler.
- Use the PageserverUtilization::score() instead of shard count when
ordering nodes in scheduling.

Q: Why did test_sharding_split_smoke need updating in this PR?
A: There's an interesting side effect during shard splits: because we do
not decrement the shard count in the utilization when we de-schedule the
shards from before the split, the controller will now prefer to pick
_different_ nodes for shards compared with which ones held secondaries
before the split. We could use our knowledge of splitting to fix up the
utilizations more actively in this situation, but I'm leaning toward
leaving the code simpler, as in practical systems the impact of one
shard on the utilization of a node should be fairly low (single digit
%).

libs/pageserver_api/src/controller_api.rs

@@ -8,6 +8,7 @@ use std::time::{Duration, Instant};
 use serde::{Deserialize, Serialize};
 use utils::id::{NodeId, TenantId};
 
+use crate::models::PageserverUtilization;
 use crate::{
     models::{ShardParameters, TenantConfig},
     shard::{ShardStripeSize, TenantShardId},
@@ -140,23 +141,11 @@ pub struct TenantShardMigrateRequest {
     pub node_id: NodeId,
 }
 
-/// Utilisation score indicating how good a candidate a pageserver
-/// is for scheduling the next tenant. See [`crate::models::PageserverUtilization`].
-/// Lower values are better.
-#[derive(Serialize, Deserialize, Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Debug)]
-pub struct UtilizationScore(pub u64);
-
-impl UtilizationScore {
-    pub fn worst() -> Self {
-        UtilizationScore(u64::MAX)
-    }
-}
-
-#[derive(Serialize, Clone, Copy, Debug)]
+#[derive(Serialize, Clone, Debug)]
 #[serde(into = "NodeAvailabilityWrapper")]
 pub enum NodeAvailability {
     // Normal, happy state
-    Active(UtilizationScore),
+    Active(PageserverUtilization),
     // Node is warming up, but we expect it to become available soon. Covers
     // the time span between the re-attach response being composed on the storage controller
     // and the first successful heartbeat after the processing of the re-attach response
@@ -195,7 +184,9 @@ impl From<NodeAvailabilityWrapper> for NodeAvailability {
         match val {
             // Assume the worst utilisation score to begin with. It will later be updated by
             // the heartbeats.
-            NodeAvailabilityWrapper::Active => NodeAvailability::Active(UtilizationScore::worst()),
+            NodeAvailabilityWrapper::Active => {
+                NodeAvailability::Active(PageserverUtilization::full())
+            }
             NodeAvailabilityWrapper::WarmingUp => NodeAvailability::WarmingUp(Instant::now()),
             NodeAvailabilityWrapper::Offline => NodeAvailability::Offline,
         }