Add compute_logical_snapshots_bytes metric (#9887)

This metric exposes the size of all non-temporary logical snapshot
files.

Signed-off-by: Tristan Partin <tristan@neon.tech>

compute/etc/neon_collector.jsonnet

@@ -6,6 +6,7 @@
     import 'sql_exporter/compute_backpressure_throttling_seconds.libsonnet',
     import 'sql_exporter/compute_current_lsn.libsonnet',
     import 'sql_exporter/compute_logical_snapshot_files.libsonnet',
+    import 'sql_exporter/compute_logical_snapshots_bytes.libsonnet',
     import 'sql_exporter/compute_max_connections.libsonnet',
     import 'sql_exporter/compute_receive_lsn.libsonnet',
     import 'sql_exporter/compute_subscriptions_count.libsonnet',

compute/etc/sql_exporter/compute_logical_snapshots_bytes.15.sql

@@ -0,0 +1,7 @@
+SELECT
+  (SELECT current_setting('neon.timeline_id')) AS timeline_id,
+  -- Postgres creates temporary snapshot files of the form %X-%X.snap.%d.tmp.
+  -- These temporary snapshot files are renamed to the actual snapshot files
+  -- after they are completely built. We only WAL-log the completely built
+  -- snapshot files
+  (SELECT COALESCE(sum(size), 0) FROM pg_ls_logicalsnapdir() WHERE name LIKE '%.snap') AS logical_snapshots_bytes;

compute/etc/sql_exporter/compute_logical_snapshots_bytes.libsonnet

@@ -0,0 +1,17 @@
+local neon = import 'neon.libsonnet';
+
+local pg_ls_logicalsnapdir = importstr 'sql_exporter/compute_logical_snapshots_bytes.15.sql';
+local pg_ls_dir = importstr 'sql_exporter/compute_logical_snapshots_bytes.sql';
+
+{
+  metric_name: 'compute_logical_snapshots_bytes',
+  type: 'gauge',
+  help: 'Size of the pg_logical/snapshots directory, not including temporary files',
+  key_labels: [
+    'timeline_id',
+  ],
+  values: [
+    'logical_snapshots_bytes',
+  ],
+  query: if neon.PG_MAJORVERSION_NUM < 15 then pg_ls_dir else pg_ls_logicalsnapdir,
+}

compute/etc/sql_exporter/compute_logical_snapshots_bytes.sql

@@ -0,0 +1,9 @@
+SELECT
+  (SELECT setting FROM pg_settings WHERE name = 'neon.timeline_id') AS timeline_id,
+  -- Postgres creates temporary snapshot files of the form %X-%X.snap.%d.tmp.
+  -- These temporary snapshot files are renamed to the actual snapshot files
+  -- after they are completely built. We only WAL-log the completely built
+  -- snapshot files
+  (SELECT COALESCE(sum((pg_stat_file('pg_logical/snapshots/' || name, missing_ok => true)).size), 0)
+    FROM (SELECT * FROM pg_ls_dir('pg_logical/snapshots') WHERE pg_ls_dir LIKE '%.snap') AS name
+  ) AS logical_snapshots_bytes;