docs/admin/monitoring: add instructions to setup outlier statistics

docs/docs/administration/monitoring.md

@@ -256,6 +256,44 @@ as well as database diagnostics.
 BEAM VM stats include detailed memory consumption breakdowns
 and a full list of running processes for example.
 
+## Postgres Statements Statistics
+
+The built-in dashboard can list the queries your instances spends the 
+most accumulative time on giving insight into potential bottlenecks
+and what might be worth optimising.
+This is the “Outliers” tab in “Ecto Stats”.  
+However for this to work you first need to enable a PostgreSQL extension
+as follows:
+
+Add the following two lines two your `postgresql.conf` (typically placed in your data dir):
+
+```
+shared_preload_libraries = 'pg_stat_statements'
+pg_stat_statements.track = all
+```
+
+Now restart PostgreSQL. Then connect to your akkoma database using `psql` and run:
+
+```sql
+CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
+```
+
+Execution time statistics will now start to be gathered.
+To get a representative sample of your instances workload you should wait a week or at least a day.
+
+These statistics are never reset automatically, but with new Akkoma releases and
+changes in the servers your instance federates with the workload will evolve.
+Thus it’s a good idea to reset this occasionally using:
+
+```sql
+-- get user oid:  SELECT oid FROM pg_roles WHERE rolname = 'akkoma';
+-- get db oid: SELECT oid FROM pg_database WHERE datname = 'akkoma';
+SELECT pg_stat_statements_reset('<akkoma user oid>'::regclass::oid, '<akkoma database oid>'::regclass::oid);
+
+-- or alternatively, to just reset stats for all users and databases:
+--   SELECT pg_stat_statements_reset();
+```
+
 ## Oban Web
 
 This too requires administrator rights to access and can be found under `/akkoma/oban` if enabled.