storage_controller: periodic pageserver heartbeats (#7092)

## Problem
If a pageserver was offline when the storage controller started, there
was no mechanism to update the
storage controller state when the pageserver becomes active.

## Summary of changes
* Add a heartbeater module. The heartbeater must be driven by an
external loop.
* Integrate the heartbeater into the service.
- Extend the types used by the service and scheduler to keep track of a
nodes' utilisation score.
- Add a background loop to drive the heartbeater and update the state
based on the deltas it generated
  - Do an initial round of heartbeats at start-up

libs/pageserver_api/src/controller_api.rs

@@ -35,7 +35,7 @@ pub struct NodeRegisterRequest {
 pub struct NodeConfigureRequest {
     pub node_id: NodeId,
 
-    pub availability: Option<NodeAvailability>,
+    pub availability: Option<NodeAvailabilityWrapper>,
     pub scheduling: Option<NodeSchedulingPolicy>,
 }
 
@@ -66,22 +66,76 @@ pub struct TenantShardMigrateRequest {
     pub node_id: NodeId,
 }
 
-#[derive(Serialize, Deserialize, Clone, Copy, Eq, PartialEq)]
+/// 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)]
+pub struct UtilizationScore(pub u64);
+
+impl UtilizationScore {
+    pub fn worst() -> Self {
+        UtilizationScore(u64::MAX)
+    }
+}
+
+#[derive(Serialize, Clone, Copy)]
+#[serde(into = "NodeAvailabilityWrapper")]
 pub enum NodeAvailability {
     // Normal, happy state
-    Active,
+    Active(UtilizationScore),
     // Offline: Tenants shouldn't try to attach here, but they may assume that their
     // secondary locations on this node still exist.  Newly added nodes are in this
     // state until we successfully contact them.
     Offline,
 }
 
+impl PartialEq for NodeAvailability {
+    fn eq(&self, other: &Self) -> bool {
+        use NodeAvailability::*;
+        matches!((self, other), (Active(_), Active(_)) | (Offline, Offline))
+    }
+}
+
+impl Eq for NodeAvailability {}
+
+// This wrapper provides serde functionality and it should only be used to
+// communicate with external callers which don't know or care about the
+// utilisation score of the pageserver it is targeting.
+#[derive(Serialize, Deserialize, Clone)]
+pub enum NodeAvailabilityWrapper {
+    Active,
+    Offline,
+}
+
+impl From<NodeAvailabilityWrapper> for NodeAvailability {
+    fn from(val: NodeAvailabilityWrapper) -> Self {
+        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::Offline => NodeAvailability::Offline,
+        }
+    }
+}
+
+impl From<NodeAvailability> for NodeAvailabilityWrapper {
+    fn from(val: NodeAvailability) -> Self {
+        match val {
+            NodeAvailability::Active(_) => NodeAvailabilityWrapper::Active,
+            NodeAvailability::Offline => NodeAvailabilityWrapper::Offline,
+        }
+    }
+}
+
 impl FromStr for NodeAvailability {
     type Err = anyhow::Error;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
-            "active" => Ok(Self::Active),
+            // This is used when parsing node configuration requests from neon-local.
+            // Assume the worst possible utilisation score
+            // and let it get updated via the heartbeats.
+            "active" => Ok(Self::Active(UtilizationScore::worst())),
             "offline" => Ok(Self::Offline),
             _ => Err(anyhow::anyhow!("Unknown availability state '{s}'")),
         }