fix: ResidentLayer::load_keys should not create INFO level span (#7174)

Since #6115 with more often used get_value_reconstruct_data and friends,
we should not have needless INFO level span creation near hot paths. In
our prod configuration, INFO spans are always created, but in practice,
very rarely anything at INFO level is logged underneath.
`ResidentLayer::load_keys` is only used during compaction so it is not
that hot, but this aligns the access paths and their span usage.

PR changes the span level to debug to align with others, and adds the
layer name to the error which was missing.

Split off from #7030.

pageserver/src/tenant/storage_layer/layer.rs

@@ -1484,7 +1484,7 @@ impl ResidentLayer {
     }
 
     /// Loads all keys stored in the layer. Returns key, lsn and value size.
-    #[tracing::instrument(skip_all, fields(layer=%self))]
+    #[tracing::instrument(level = tracing::Level::DEBUG, skip_all, fields(layer=%self))]
     pub(crate) async fn load_keys<'a>(
         &'a self,
         ctx: &RequestContext,
@@ -1504,9 +1504,9 @@ impl ResidentLayer {
                 // while it's being held.
                 delta_layer::DeltaLayerInner::load_keys(d, ctx)
                     .await
-                    .context("Layer index is corrupted")
+                    .with_context(|| format!("Layer index is corrupted for {self}"))
             }
-            Image(_) => anyhow::bail!("cannot load_keys on a image layer"),
+            Image(_) => anyhow::bail!(format!("cannot load_keys on a image layer {self}")),
         }
     }