core: Make advertise_addr of Node optional (#618)

The semantics here changed slightly, in that a `None` address means that
the node is known but not currently connected.

crates/client-api/src/routes/prometheus.rs

@@ -22,7 +22,9 @@ pub async fn get_sd_config<S: ControlStateReadAccess>(
     let labels = HashMap::new();
 
     for node in nodes {
-        targets.push(node.advertise_addr);
+        if let Some(addr) = node.advertise_addr {
+            targets.push(addr);
+        }
     }
 
     let sd_config = SDConfig { targets, labels };

crates/core/src/messages/control_db.rs

@@ -50,10 +50,12 @@ pub struct DatabaseInstanceStatus {
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Node {
     pub id: u64,
+    /// If `true`, no new user databases will be scheduled on this node.
     pub unschedulable: bool,
-    /// TODO: It's unclear if this should be in here since it's arguably status
-    /// rather than part of the configuration kind of. I dunno.
-    pub advertise_addr: String,
+    /// The hostname this node is reachable at.
+    ///
+    /// If `None`, the node is not currently live.
+    pub advertise_addr: Option<String>,
 }
 #[derive(Clone, PartialEq, Serialize, Deserialize)]
 pub struct NodeStatus {

crates/standalone/src/lib.rs

@@ -220,7 +220,7 @@ impl spacetimedb_client_api::ControlStateReadAccess for StandaloneEnv {
             return Ok(Some(Node {
                 id: 0,
                 unschedulable: false,
-                advertise_addr: "node:80".into(),
+                advertise_addr: Some("node:80".to_owned()),
             }));
         }
         Ok(None)