Postgres: Prioritize collection of size metrics on larger tables

The `max_relations` parameter
(https://github.com/DataDog/integrations-core/blob/cf923f4aa3e63d/postgres/datadog_checks/postgres/data/conf.yaml.example#L160-L163)
limits the number of relations that metrics are collected on.
Previously, no order was specified, which meant that the metrics that
were reported depedended on the vagaries of Postgres. This change orders
the query by total table size because people are more likely to care
about size-related metrics for larger tables than for smaller ones.
There are exceptions, but even so, collecting metrics for the largest N
tables seems more useful than collecting metrics for an unspecified
subset of N tables.

postgres/datadog_checks/postgres/relationsmanager.py

@@ -110,7 +110,8 @@
 WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND
   nspname !~ '^pg_toast' AND
   relkind = 'r' AND
-  {relations}""",
+  {relations}
+ORDER BY pg_total_relation_size(C.oid) DESC""",
 }
 
 # The pg_statio_all_tables view will contain one row for each table in the current database,