externalize statistics on LFC cache usage (#6906)

## Problem

Customers should be able to determine the size of their workload's
working set to right size their compute.
Since Neon uses Local file cache (LFC) instead of shared buffers on
bigger compute nodes to cache pages we need to externalize a means to
determine LFC hit ratio in addition to shared buffer hit ratio.

Currently the following end user documentation
fb7cd3af0e/content/docs/manage/endpoints.md (L137)
is wrong because it describes how to right size a compute node based on
shared buffer hit ratio.

Note that the existing functionality in extension "neon" is NOT
available to end users but only to superuser / cloud_admin.

## Summary of changes

- externalize functions and views in neon extension to end users
- introduce a new view `NEON_STAT_FILE_CACHE` with the following DDL

```sql
CREATE OR REPLACE VIEW NEON_STAT_FILE_CACHE AS 
   WITH lfc_stats AS (
   SELECT 
     stat_name, 
     count
   FROM neon_get_lfc_stats() AS t(stat_name text, count bigint)
   ),
   lfc_values AS (
   SELECT 
     MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE NULL END) AS file_cache_misses,
     MAX(CASE WHEN stat_name = 'file_cache_hits'   THEN count ELSE NULL END) AS file_cache_hits,
     MAX(CASE WHEN stat_name = 'file_cache_used'   THEN count ELSE NULL END) AS file_cache_used,
     MAX(CASE WHEN stat_name = 'file_cache_writes' THEN count ELSE NULL END) AS file_cache_writes,
     -- Calculate the file_cache_hit_ratio within the same CTE for simplicity
     CASE 
        WHEN MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE 0 END) + MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END) = 0 THEN NULL
        ELSE ROUND((MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END)::DECIMAL / 
        (MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END) + MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE 0 END))) * 100, 2)
     END AS file_cache_hit_ratio
   FROM lfc_stats
   )
SELECT file_cache_misses, file_cache_hits, file_cache_used, file_cache_writes, file_cache_hit_ratio from lfc_values;
```

This view can be used by an end user as follows:

```sql
CREATE EXTENSION NEON;
SELECT * from neon. NEON_STAT_FILE_CACHE"
```

The output looks like the following:

```
select * from NEON_STAT_FILE_CACHE;
 file_cache_misses | file_cache_hits | file_cache_used | file_cache_writes | file_cache_hit_ratio  
-------------------+-----------------+-----------------+-------------------+----------------------
           2133643 |       108999742 |             607 |          10767410 |                98.08
(1 row)

```

## Checklist before requesting a review

- [x ] I have performed a self-review of my code.
- [x ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [x ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

pgxn/neon/Makefile

@@ -21,7 +21,7 @@ SHLIB_LINK_INTERNAL = $(libpq)
 SHLIB_LINK = -lcurl
 
 EXTENSION = neon
-DATA = neon--1.0.sql neon--1.0--1.1.sql
+DATA = neon--1.0.sql neon--1.0--1.1.sql neon--1.1--1.2.sql
 PGFILEDESC = "neon - cloud storage for PostgreSQL"
 
 EXTRA_CLEAN = \

pgxn/neon/neon--1.1--1.2.sql

@@ -0,0 +1,29 @@
+\echo Use "ALTER EXTENSION neon UPDATE TO '1.2'" to load this file. \quit
+
+-- Create a convenient view similar to pg_stat_database
+-- that exposes all lfc stat values in one row.
+CREATE OR REPLACE VIEW NEON_STAT_FILE_CACHE AS 
+   WITH lfc_stats AS (
+   SELECT 
+     stat_name, 
+     count
+   FROM neon_get_lfc_stats() AS t(stat_name text, count bigint)
+   ),
+   lfc_values AS (
+   SELECT 
+     MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE NULL END) AS file_cache_misses,
+     MAX(CASE WHEN stat_name = 'file_cache_hits'   THEN count ELSE NULL END) AS file_cache_hits,
+     MAX(CASE WHEN stat_name = 'file_cache_used'   THEN count ELSE NULL END) AS file_cache_used,
+     MAX(CASE WHEN stat_name = 'file_cache_writes' THEN count ELSE NULL END) AS file_cache_writes,
+     -- Calculate the file_cache_hit_ratio within the same CTE for simplicity
+     CASE 
+        WHEN MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE 0 END) + MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END) = 0 THEN NULL
+        ELSE ROUND((MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END)::DECIMAL / 
+        (MAX(CASE WHEN stat_name = 'file_cache_hits' THEN count ELSE 0 END) + MAX(CASE WHEN stat_name = 'file_cache_misses' THEN count ELSE 0 END))) * 100, 2)
+     END AS file_cache_hit_ratio
+   FROM lfc_stats
+   )
+SELECT file_cache_misses, file_cache_hits, file_cache_used, file_cache_writes, file_cache_hit_ratio from lfc_values;
+
+-- externalize the view to all users in role pg_monitor
+GRANT SELECT ON NEON_STAT_FILE_CACHE TO PG_MONITOR;

pgxn/neon/neon.control

@@ -1,5 +1,6 @@
 # neon extension
 comment = 'cloud storage for PostgreSQL'
-default_version = '1.1'
+default_version = '1.2'
 module_pathname = '$libdir/neon'
 relocatable = true
+trusted = true

test_runner/regress/test_neon_extension.py

@@ -1,5 +1,6 @@
 from contextlib import closing
 
+from fixtures.log_helper import log
 from fixtures.neon_fixtures import NeonEnvBuilder
 
 
@@ -22,4 +23,9 @@ def test_neon_extension(neon_env_builder: NeonEnvBuilder):
             # IMPORTANT:
             # If the version has changed, the test should be updated.
             # Ensure that the default version is also updated in the neon.control file
-            assert cur.fetchone() == ("1.1",)
+            assert cur.fetchone() == ("1.2",)
+            cur.execute("SELECT * from neon.NEON_STAT_FILE_CACHE")
+            res = cur.fetchall()
+            log.info(res)
+            assert len(res) == 1
+            assert len(res[0]) == 5