Enable pg_stat_statements on the authentik postgres cluster (#475)

The cluster preloads no statement-statistics library, so there is no per-query cost attribution in postgres and slow query paths have to be inferred from application-side metrics instead of read straight out of the database.

- preload `pg_stat_statements`
- set `pg_stat_statements.max` and `.track`, which is what makes CNPG manage the extension and create it in every database

Requires a postgres restart. Stacked on `benvin/authentik-cnpg-resources`.

Reviewed-on: #475
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/authentik/cnpg_cluster.yaml

@@ -81,6 +81,11 @@ spec:
       max_parallel_workers: "16"
       max_replication_slots: "16"
       max_worker_processes: "16"
+      # A pg_stat_statements.* parameter is what makes CNPG treat the extension as
+      # managed and run CREATE EXTENSION in every database; preloading alone does
+      # not create it.
+      pg_stat_statements.max: "10000"
+      pg_stat_statements.track: top
       shared_buffers: 512MB
       shared_memory_type: mmap
       ssl_max_protocol_version: TLSv1.3
@@ -90,6 +95,9 @@ spec:
       wal_log_hints: "on"
       wal_receiver_timeout: 5s
       wal_sender_timeout: 5s
+    # CNPG merges this with the libraries it manages itself.
+    shared_preload_libraries:
+      - pg_stat_statements
     syncReplicaElectionConstraint:
       enabled: false
   primaryUpdateMethod: restart