Monitor Postgres TOAST oid exhaustion
Postgres toast tables can exhaust their 32 bit keyspace of OIDs if they are filled with 2^32 rows. Even before that point, insert performance can rapidly decrease as the keyspace fills - postgres tries the next OID from a global, wrapping counter on every insert and simply retries with another if it sees a collision, so with many collisions insert performance will tank.
It's difficult to fix this sort of exhaustion - it would require moving columns to different tables, deleting data, or partitioning - so it would be very useful to have early alerting on it.
Add some sort of alerting for this condition, so that we know long before we're approaching saturation.