pageserver: add timeline_id span when freezing layers (#12572)

## Problem

We don't log the timeline ID when rolling ephemeral layers during
housekeeping.

Resolves [LKB-179](https://databricks.atlassian.net/browse/LKB-179)

## Summary of changes

Add a span with timeline ID when calling `maybe_freeze_ephemeral_layer`
from the housekeeping loop.

We don't instrument the function itself, since future callers may not
have a span including the tenant_id already, but we don't want to
duplicate the tenant_id for these spans.

pageserver/src/tenant.rs

@@ -3393,7 +3393,13 @@ impl TenantShard {
                 .collect_vec();
 
             for timeline in timelines {
-                timeline.maybe_freeze_ephemeral_layer().await;
+                // Include a span with the timeline ID. The parent span already has the tenant ID.
+                let span =
+                    info_span!("maybe_freeze_ephemeral_layer", timeline_id = %timeline.timeline_id);
+                timeline
+                    .maybe_freeze_ephemeral_layer()
+                    .instrument(span)
+                    .await;
             }
         }
 

pageserver/src/tenant/timeline.rs

@@ -1893,6 +1893,8 @@ impl Timeline {
     // an ephemeral layer open forever when idle.  It also freezes layers if the global limit on
     // ephemeral layer bytes has been breached.
     pub(super) async fn maybe_freeze_ephemeral_layer(&self) {
+        debug_assert_current_span_has_tenant_and_timeline_id();
+
         let Ok(mut write_guard) = self.write_lock.try_lock() else {
             // If the write lock is held, there is an active wal receiver: rolling open layers
             // is their responsibility while they hold this lock.