add info around pgbouncer columns avoided

Author: benjaminjb

internal/collector/pgbouncer_metrics_queries.yaml

@@ -11,7 +11,9 @@
         attribute_columns: ["database", "user", "state", "application_name", "link"]
         description: "Current waiting time in seconds"
 
-  # NOTE: Avoid collecting "host" column because it can be null; the collector will warn against null.
+  # NOTE: Avoid collecting/using "host", "force_user", and "pool_mode" columns because they 
+  # can be NULL; the collector will warn against NULL even when not used. But it will emit
+  # an error log if those columns are used.
   # The host column should always point either to pgBouncer's virtual database (the null case) or to the primary.
   - sql: "SHOW DATABASES"
     metrics: