internal-devx: pageserver future layers (#5092)

I've personally forgotten why/how can we have future layers during
reconciliation. Adds `#[cfg(feature = "testing")]` logging when we
upload such index_part.json, with a cross reference to where the cleanup
happens.

Latest private slack thread:
https://neondb.slack.com/archives/C033RQ5SPDH/p1692879032573809?thread_ts=1692792276.173979&cid=C033RQ5SPDH

Builds upon #5074. Should had been considered on #4837.

pageserver/src/tenant/remote_timeline_client.rs

@@ -1076,6 +1076,15 @@ impl RemoteTimelineClient {
                     .await
                 }
                 UploadOp::UploadMetadata(ref index_part, _lsn) => {
+                    let mention_having_future_layers = if cfg!(feature = "testing") {
+                        index_part
+                            .layer_metadata
+                            .keys()
+                            .any(|x| x.is_in_future(*_lsn))
+                    } else {
+                        false
+                    };
+
                     let res = upload::upload_index_part(
                         self.conf,
                         &self.storage_impl,
@@ -1093,6 +1102,10 @@ impl RemoteTimelineClient {
                     .await;
                     if res.is_ok() {
                         self.update_remote_physical_size_gauge(Some(index_part));
+                        if mention_having_future_layers {
+                            // find rationale near crate::tenant::timeline::init::cleanup_future_layer
+                            tracing::info!(disk_consistent_lsn=%_lsn, "uploaded an index_part.json with future layers -- this is ok! if shutdown now, expect future layer cleanup");
+                        }
                     }
                     res
                 }

pageserver/src/tenant/storage_layer/filename.rs

@@ -212,7 +212,7 @@ pub enum LayerFileName {
 }
 
 impl LayerFileName {
-    pub fn file_name(&self) -> String {
+    pub(crate) fn file_name(&self) -> String {
         self.to_string()
     }
 
@@ -274,8 +274,8 @@ impl serde::Serialize for LayerFileName {
         S: serde::Serializer,
     {
         match self {
-            Self::Image(fname) => serializer.serialize_str(&fname.to_string()),
-            Self::Delta(fname) => serializer.serialize_str(&fname.to_string()),
+            Self::Image(fname) => serializer.collect_str(fname),
+            Self::Delta(fname) => serializer.collect_str(fname),
         }
     }
 }