pageserver: tweak logging of "became visible" for layers (#9224)

## Problem

Recent change to avoid the "became visible" log messages from certain
tasks missed a task: the logical size calculation that happens as a
child of synthetic size calculation.

Related: https://github.com/neondatabase/neon/issues/9058

## Summary of changes

- Add OnDemandLogicalSize to the list of permitted tasks for reads
making a covered layer visible
- Tweak the log message to use layer name instead of key: this is more
terse, and easier to use when debugging, as one can search for it
elsewhere to see when the layer was written/downloaded etc.

pageserver/src/tenant/storage_layer/layer.rs

@@ -442,11 +442,13 @@ impl Layer {
             // Visibility was modified to Visible: maybe log about this
             match ctx.task_kind() {
                 TaskKind::CalculateSyntheticSize
+                | TaskKind::OndemandLogicalSizeCalculation
                 | TaskKind::GarbageCollector
                 | TaskKind::MgmtRequest => {
                     // This situation is expected in code paths do binary searches of the LSN space to resolve
                     // an LSN to a timestamp, which happens during GC, during GC cutoff calculations in synthetic size,
-                    // and on-demand for certain HTTP API requests.
+                    // and on-demand for certain HTTP API requests. On-demand logical size calculation is also included
+                    // because it is run as a sub-task of synthetic size.
                 }
                 _ => {
                     // In all other contexts, it is unusual to do I/O involving layers which are not visible at
@@ -457,7 +459,7 @@ impl Layer {
                     // which was covered by a concurrent compaction.
                     tracing::info!(
                         "Layer {} became visible as a result of access",
-                        self.0.desc.key()
+                        self.0.desc.layer_name()
                     );
                 }
             }