Persist pg_stat information in pageserver (#6560)

## Problem

Statistic is saved in local file and so lost on compute restart.

Persist in in page server using the same AUX file mechanism used for
replication slots

See more about motivation in
https://neondb.slack.com/archives/C04DGM6SMTM/p1703077676522789

## Summary of changes

Persist postal file using AUX mechanism


Postgres PRs:
https://github.com/neondatabase/postgres/pull/547
https://github.com/neondatabase/postgres/pull/446
https://github.com/neondatabase/postgres/pull/445

Related to #6684 and #6228

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pageserver/src/pgdatadir_mapping.rs

@@ -45,7 +45,7 @@ use std::ops::ControlFlow;
 use std::ops::Range;
 use strum::IntoEnumIterator;
 use tokio_util::sync::CancellationToken;
-use tracing::{debug, trace, warn};
+use tracing::{debug, info, trace, warn};
 use utils::bin_ser::DeserializeError;
 use utils::pausable_failpoint;
 use utils::{bin_ser::BeSer, lsn::Lsn};
@@ -2264,6 +2264,13 @@ impl DatadirModification<'_> {
                 self.tline.aux_file_size_estimator.on_add(content.len());
                 new_files.push((path, content));
             }
+            // Compute may request delete of old version of pgstat AUX file if new one exceeds size limit.
+            // Compute doesn't know if previous version of this file exists or not, so
+            // attempt to delete non-existing file can cause this message.
+            // To avoid false alarms, log it as info rather than warning.
+            (None, true) if path.starts_with("pg_stat/") => {
+                info!("removing non-existing pg_stat file: {}", path)
+            }
             (None, true) => warn!("removing non-existing aux file: {}", path),
         }
         let new_val = aux_file::encode_file_value(&new_files)?;