callers of task_mgr::spawn: some top-level async blocks were missing tenant/timeline id (#4283)

Looking at logs from staging and prod, I found there are a bunch of log
lines without tenant / timeline context.

Manully walk through all task_mgr::spawn lines and fix that using the
least amount of work required.

While doing it, remove some redundant `shutting down` messages. 

refs https://github.com/neondatabase/neon/issues/4222

pageserver/src/disk_usage_eviction_task.rs

@@ -110,7 +110,6 @@ pub fn launch_disk_usage_global_eviction_task(
 
             disk_usage_eviction_task(&state, task_config, storage, &conf.tenants_path(), cancel)
                 .await;
-            info!("disk usage based eviction task finishing");
             Ok(())
         },
     );
@@ -126,13 +125,16 @@ async fn disk_usage_eviction_task(
     tenants_dir: &Path,
     cancel: CancellationToken,
 ) {
+    scopeguard::defer! {
+        info!("disk usage based eviction task finishing");
+    };
+
     use crate::tenant::tasks::random_init_delay;
     {
         if random_init_delay(task_config.period, &cancel)
             .await
             .is_err()
         {
-            info!("shutting down");
             return;
         }
     }
@@ -167,7 +169,6 @@ async fn disk_usage_eviction_task(
         tokio::select! {
             _ = tokio::time::sleep_until(sleep_until) => {},
             _ = cancel.cancelled() => {
-                info!("shutting down");
                 break
             }
         }

pageserver/src/tenant/timeline/eviction_task.rs

@@ -70,7 +70,6 @@ impl Timeline {
                 };
 
                 self_clone.eviction_task(cancel).await;
-                info!("eviction task finishing");
                 Ok(())
             },
         );
@@ -78,6 +77,9 @@ impl Timeline {
 
     #[instrument(skip_all, fields(tenant_id = %self.tenant_id, timeline_id = %self.timeline_id))]
     async fn eviction_task(self: Arc<Self>, cancel: CancellationToken) {
+        scopeguard::defer! {
+            info!("eviction task finishing");
+        }
         use crate::tenant::tasks::random_init_delay;
         {
             let policy = self.get_eviction_policy();
@@ -86,7 +88,6 @@ impl Timeline {
                 EvictionPolicy::NoEviction => Duration::from_secs(10),
             };
             if random_init_delay(period, &cancel).await.is_err() {
-                info!("shutting down");
                 return;
             }
         }
@@ -101,7 +102,6 @@ impl Timeline {
                 ControlFlow::Continue(sleep_until) => {
                     tokio::select! {
                         _ = cancel.cancelled() => {
-                            info!("shutting down");
                             break;
                         }
                         _ = tokio::time::sleep_until(sleep_until) => { }

pageserver/src/tenant/timeline/walreceiver/walreceiver_connection.rs

@@ -71,6 +71,8 @@ pub(super) async fn handle_walreceiver_connection(
     ctx: RequestContext,
     node: NodeId,
 ) -> anyhow::Result<()> {
+    debug_assert_current_span_has_tenant_and_timeline_id();
+
     WALRECEIVER_STARTED_CONNECTIONS.inc();
 
     // Connect to the database in replication mode.
@@ -140,6 +142,9 @@ pub(super) async fn handle_walreceiver_connection(
             }
             Ok(())
         }
+        // Enrich the log lines emitted by this closure with meaningful context.
+        // TODO: technically, this task outlives the surrounding function, so, the
+        // spans won't be properly nested.
         .instrument(tracing::info_span!("poller")),
     );