Support OldestReplicationSlotLag on PostgreSQL

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Describe the Feature

Support an RDS alarm on OldestReplicationSlotLag.

Oldest Replication Slot Lag (MB):

The lagging size of the replica lagging the most in terms of write-ahead log (WAL) data received. Applies to PostgreSQL.

We lose an RDS alarm on OldestReplicationSlotLag when changing our module:

• from https://github.com/BetterWorks/terraform-aws-rds-cloudwatch-sns-alarms.git?ref=tags/0.3.6
• to https://github.com/cloudposse/terraform-aws-rds-cloudwatch-sns-alarms.git?ref=tags/0.3.1

See

• https://github.com/BetterWorks/terraform-aws-rds-cloudwatch-sns-alarms/blob/master/alarms.tf#L128
• https://github.com/cloudposse/terraform-aws-rds-cloudwatch-sns-alarms/commit/3b67e0959695533a28a79242c2d9bc5d09b24394

Related refs:

• https://dba.stackexchange.com/questions/195040/spike-in-aws-rds-oldest-replication-slot-lag
• https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-metrics.html

Expected Behavior

When an RDS is using PostgreSQL, the monitoring includes OldestReplicationSlotLag.