pageserver: fix a warning on secondary mode downloads after evictions (#7877)

## Problem

In 4ce6e2d2fc we added a warning when progress stats don't look right at
the end of a secondary download pass.

This `Correcting drift in progress stats` warning fired in staging on a
pageserver that had been doing some disk usage eviction.

The impact is low because in the same place we log the warning, we also
fix up the progress values.

## Summary of changes

- When we skip downloading a layer because it was recently evicted,
update the progress stats to ensure they still reach a clean complete
state at the end of a download pass.
- Also add a log for evicting secondary location layers, for symmetry
with attached locations, so that we can clearly see when eviction has
happened for a particular tenant's layers when investigating issues.

This is a point fix -- the code would also benefit from being refactored
so that there is some "download result" type with a Skip variant, to
ensure that we are updating the progress stats uniformly for those
cases.

pageserver/src/tenant/secondary.rs

@@ -187,6 +187,7 @@ impl SecondaryTenant {
         };
 
         let now = SystemTime::now();
+        tracing::info!("Evicting secondary layer");
 
         let this = self.clone();
 

pageserver/src/tenant/secondary/downloader.rs

@@ -909,6 +909,7 @@ impl<'a> TenantDownloader<'a> {
                         strftime(&layer.access_time),
                         strftime(evicted_at)
                     );
+                    self.skip_layer(layer);
                     continue;
                 }
             }
@@ -963,6 +964,15 @@ impl<'a> TenantDownloader<'a> {
         Ok(())
     }
 
+    /// Call this during timeline download if a layer will _not_ be downloaded, to update progress statistics
+    fn skip_layer(&self, layer: HeatMapLayer) {
+        let mut progress = self.secondary_state.progress.lock().unwrap();
+        progress.layers_total = progress.layers_total.saturating_sub(1);
+        progress.bytes_total = progress
+            .bytes_total
+            .saturating_sub(layer.metadata.file_size);
+    }
+
     async fn download_layer(
         &self,
         tenant_shard_id: &TenantShardId,
@@ -1012,13 +1022,7 @@ impl<'a> TenantDownloader<'a> {
                     "Skipped downloading missing layer {}, raced with compaction/gc?",
                     layer.name
                 );
-
-                // If the layer is 404, adjust the progress statistics to reflect that we will not download it.
-                let mut progress = self.secondary_state.progress.lock().unwrap();
-                progress.layers_total = progress.layers_total.saturating_sub(1);
-                progress.bytes_total = progress
-                    .bytes_total
-                    .saturating_sub(layer.metadata.file_size);
+                self.skip_layer(layer);
 
                 return Ok(None);
             }